Add PerplexityLLMAdapter to enforce Perplexity's message ordering constraints

Perplexity's API is stricter than OpenAI about conversation history:
- Requires strict alternation between user/tool and assistant messages
- Disallows system messages except as the initial message
- Requires the last message to be user or tool

The new adapter transforms messages before sending to satisfy all three
constraints: merging consecutive initial system messages, converting
non-initial system to user, merging consecutive same-role messages, and
removing trailing assistant messages.

Also adds dual-system-instruction warnings to Cerebras, Fireworks,
Mistral, Perplexity, and SambaNova services (matching the existing
BaseOpenAILLMService pattern), and updates the warning text in
BaseOpenAILLMService to be more descriptive.

tests/test_get_llm_invocation_params.py

@@ -48,6 +48,7 @@ from pipecat.adapters.services.anthropic_adapter import AnthropicLLMAdapter
 from pipecat.adapters.services.bedrock_adapter import AWSBedrockLLMAdapter
 from pipecat.adapters.services.gemini_adapter import GeminiLLMAdapter
 from pipecat.adapters.services.open_ai_adapter import OpenAILLMAdapter
+from pipecat.adapters.services.perplexity_adapter import PerplexityLLMAdapter
 from pipecat.processors.aggregators.llm_context import (
     LLMContext,
     LLMStandardMessage,
@@ -992,5 +993,201 @@ class TestAWSBedrockGetLLMInvocationParams(unittest.TestCase):
         self.assertEqual(len(params["messages"]), 0)
 
 
+class TestPerplexityGetLLMInvocationParams(unittest.TestCase):
+    def setUp(self) -> None:
+        """Sets up a common adapter instance for all tests."""
+        self.adapter = PerplexityLLMAdapter()
+
+    def test_standard_messages_pass_through(self):
+        """Test that a valid [user, assistant, user] sequence passes through unchanged."""
+        messages: list[LLMStandardMessage] = [
+            {"role": "user", "content": "Hello"},
+            {"role": "assistant", "content": "Hi there!"},
+            {"role": "user", "content": "How are you?"},
+        ]
+
+        context = LLMContext(messages=messages)
+        params = self.adapter.get_llm_invocation_params(context)
+
+        self.assertEqual(len(params["messages"]), 3)
+        self.assertEqual(params["messages"][0]["role"], "user")
+        self.assertEqual(params["messages"][0]["content"], "Hello")
+        self.assertEqual(params["messages"][1]["role"], "assistant")
+        self.assertEqual(params["messages"][1]["content"], "Hi there!")
+        self.assertEqual(params["messages"][2]["role"], "user")
+        self.assertEqual(params["messages"][2]["content"], "How are you?")
+
+    def test_initial_system_message_preserved(self):
+        """Test that a valid [system, user, assistant, user] sequence passes through unchanged."""
+        messages: list[LLMStandardMessage] = [
+            {"role": "system", "content": "You are a helpful assistant."},
+            {"role": "user", "content": "Hello"},
+            {"role": "assistant", "content": "Hi!"},
+            {"role": "user", "content": "Bye"},
+        ]
+
+        context = LLMContext(messages=messages)
+        params = self.adapter.get_llm_invocation_params(context)
+
+        self.assertEqual(len(params["messages"]), 4)
+        self.assertEqual(params["messages"][0]["role"], "system")
+        self.assertEqual(params["messages"][0]["content"], "You are a helpful assistant.")
+        self.assertEqual(params["messages"][1]["role"], "user")
+        self.assertEqual(params["messages"][2]["role"], "assistant")
+        self.assertEqual(params["messages"][3]["role"], "user")
+
+    def test_consecutive_same_role_messages_merged(self):
+        """Test that consecutive user messages are merged into list-of-dicts content."""
+        messages: list[LLMStandardMessage] = [
+            {"role": "user", "content": "First message"},
+            {"role": "user", "content": "Second message"},
+            {"role": "assistant", "content": "Response"},
+            {"role": "user", "content": "Third message"},
+        ]
+
+        context = LLMContext(messages=messages)
+        params = self.adapter.get_llm_invocation_params(context)
+
+        self.assertEqual(len(params["messages"]), 3)
+
+        # First message should be merged users
+        merged = params["messages"][0]
+        self.assertEqual(merged["role"], "user")
+        self.assertIsInstance(merged["content"], list)
+        self.assertEqual(len(merged["content"]), 2)
+        self.assertEqual(merged["content"][0]["type"], "text")
+        self.assertEqual(merged["content"][0]["text"], "First message")
+        self.assertEqual(merged["content"][1]["type"], "text")
+        self.assertEqual(merged["content"][1]["text"], "Second message")
+
+        self.assertEqual(params["messages"][1]["role"], "assistant")
+        self.assertEqual(params["messages"][2]["role"], "user")
+
+    def test_non_initial_system_converted_to_user(self):
+        """Test that non-initial system messages are converted to user and merged with adjacent user."""
+        messages: list[LLMStandardMessage] = [
+            {"role": "system", "content": "You are helpful."},
+            {"role": "user", "content": "Hello"},
+            {"role": "assistant", "content": "Hi!"},
+            {"role": "system", "content": "Be concise."},
+            {"role": "user", "content": "Tell me about Python."},
+        ]
+
+        context = LLMContext(messages=messages)
+        params = self.adapter.get_llm_invocation_params(context)
+
+        # system(initial), user, assistant, merged(system→user + user)
+        self.assertEqual(len(params["messages"]), 4)
+        self.assertEqual(params["messages"][0]["role"], "system")
+        self.assertEqual(params["messages"][1]["role"], "user")
+        self.assertEqual(params["messages"][2]["role"], "assistant")
+
+        # The converted system→user and the following user should be merged
+        merged = params["messages"][3]
+        self.assertEqual(merged["role"], "user")
+        self.assertIsInstance(merged["content"], list)
+        self.assertEqual(len(merged["content"]), 2)
+        self.assertEqual(merged["content"][0]["text"], "Be concise.")
+        self.assertEqual(merged["content"][1]["text"], "Tell me about Python.")
+
+    def test_multiple_system_messages_at_start_merged(self):
+        """Test that multiple consecutive system messages at start are merged into one."""
+        messages: list[LLMStandardMessage] = [
+            {"role": "system", "content": "You are a helpful assistant."},
+            {"role": "system", "content": "Always be polite."},
+            {"role": "user", "content": "Hello"},
+        ]
+
+        context = LLMContext(messages=messages)
+        params = self.adapter.get_llm_invocation_params(context)
+
+        self.assertEqual(len(params["messages"]), 2)
+
+        # First message should be merged system
+        system_msg = params["messages"][0]
+        self.assertEqual(system_msg["role"], "system")
+        self.assertIsInstance(system_msg["content"], list)
+        self.assertEqual(len(system_msg["content"]), 2)
+        self.assertEqual(system_msg["content"][0]["text"], "You are a helpful assistant.")
+        self.assertEqual(system_msg["content"][1]["text"], "Always be polite.")
+
+        self.assertEqual(params["messages"][1]["role"], "user")
+        self.assertEqual(params["messages"][1]["content"], "Hello")
+
+    def test_trailing_assistant_removed(self):
+        """Test that a trailing assistant message is removed."""
+        messages: list[LLMStandardMessage] = [
+            {"role": "user", "content": "Hello"},
+            {"role": "assistant", "content": "Hi there!"},
+        ]
+
+        context = LLMContext(messages=messages)
+        params = self.adapter.get_llm_invocation_params(context)
+
+        self.assertEqual(len(params["messages"]), 1)
+        self.assertEqual(params["messages"][0]["role"], "user")
+        self.assertEqual(params["messages"][0]["content"], "Hello")
+
+    def test_only_system_message_converted_to_user(self):
+        """Test that a single system message is converted to user role."""
+        messages: list[LLMStandardMessage] = [
+            {"role": "system", "content": "You are a helpful assistant."},
+        ]
+
+        context = LLMContext(messages=messages)
+        params = self.adapter.get_llm_invocation_params(context)
+
+        self.assertEqual(len(params["messages"]), 1)
+        self.assertEqual(params["messages"][0]["role"], "user")
+        self.assertEqual(params["messages"][0]["content"], "You are a helpful assistant.")
+
+    def test_consecutive_assistants_merged_then_trailing_removed(self):
+        """Test that consecutive assistant messages are merged, then trailing assistant is removed."""
+        messages: list[LLMStandardMessage] = [
+            {"role": "user", "content": "Hello"},
+            {"role": "assistant", "content": "First response"},
+            {"role": "assistant", "content": "Second response"},
+        ]
+
+        context = LLMContext(messages=messages)
+        params = self.adapter.get_llm_invocation_params(context)
+
+        # After merging assistants we get [user, assistant(merged)], then trailing
+        # assistant is removed, leaving just [user]
+        self.assertEqual(len(params["messages"]), 1)
+        self.assertEqual(params["messages"][0]["role"], "user")
+        self.assertEqual(params["messages"][0]["content"], "Hello")
+
+    def test_tool_messages_preserved(self):
+        """Test that tool messages pass through without modification."""
+        messages: list[LLMStandardMessage] = [
+            {"role": "user", "content": "What's the weather?"},
+            {
+                "role": "assistant",
+                "content": "Let me check.",
+                "tool_calls": [{"id": "1", "function": {"name": "get_weather", "arguments": "{}"}}],
+            },
+            {"role": "tool", "content": "Sunny, 72F", "tool_call_id": "1"},
+            {"role": "user", "content": "Thanks!"},
+        ]
+
+        context = LLMContext(messages=messages)
+        params = self.adapter.get_llm_invocation_params(context)
+
+        self.assertEqual(len(params["messages"]), 4)
+        self.assertEqual(params["messages"][0]["role"], "user")
+        self.assertEqual(params["messages"][1]["role"], "assistant")
+        self.assertEqual(params["messages"][2]["role"], "tool")
+        self.assertEqual(params["messages"][2]["content"], "Sunny, 72F")
+        self.assertEqual(params["messages"][3]["role"], "user")
+
+    def test_empty_messages(self):
+        """Test that empty messages list returns empty."""
+        context = LLMContext(messages=[])
+        params = self.adapter.get_llm_invocation_params(context)
+
+        self.assertEqual(params["messages"], [])
+
+
 if __name__ == "__main__":
     unittest.main()