Enhance greeting context management in Brain classes

- Introduce greeting context handling in BaseBrain and WorkflowBrain to manage assistant greetings effectively.
- Implement prepare_greeting_context method to add greeting messages to the local context while preserving playback order.
- Update pipeline event handling to ensure greeting timestamps are maintained until the client is ready.
- Enhance tests to verify the correct behavior of greeting context management in various scenarios.

backend/services/brains/base.py

@@ -19,6 +19,20 @@ from pipecat.processors.aggregators.llm_context import LLMContext
 from pipecat.processors.frame_processor import FrameProcessor
 
 
+GREETING_CONTEXT_MARKER = "[会话事实：助手开场白已播放]"
+
+
+def greeting_context_message(greeting: str) -> dict[str, str] | None:
+    """Represent spoken greeting without starting model history as assistant."""
+    content = greeting.strip()
+    if not content:
+        return None
+    return {
+        "role": "system",
+        "content": f"{GREETING_CONTEXT_MARKER}\n{content}",
+    }
+
+
 @dataclass(frozen=True)
 class BrainSpec:
     """Static capabilities used by validation and runtime dispatch."""
@@ -86,6 +100,27 @@ class BaseBrain:
     async def on_connected(self) -> None:
         """Handle a connected client after the common greeting is queued."""
 
+    def prepare_greeting_context(
+        self,
+        greeting: str,
+        context: LLMContext,
+    ) -> dict[str, str] | None:
+        """Add a provider-safe fact describing the greeting to local context."""
+        if not self.spec.owns_context:
+            return None
+        message = greeting_context_message(greeting)
+        if message is None:
+            return None
+        messages = context.get_messages()
+        messages[:] = [
+            item
+            for item in messages
+            if GREETING_CONTEXT_MARKER not in str(item.get("content") or "")
+        ]
+        insert_at = 1 if messages and messages[0].get("role") == "system" else 0
+        messages.insert(insert_at, message)
+        return message
+
     async def on_client_ready(self) -> None:
         """Replay client-visible state after its app message channel is ready."""
 
@@ -129,6 +164,12 @@ class Brain(Protocol):
 
     async def on_connected(self) -> None: ...
 
+    def prepare_greeting_context(
+        self,
+        greeting: str,
+        context: LLMContext,
+    ) -> dict[str, str] | None: ...
+
     async def on_client_ready(self) -> None: ...
 
     def record_user_message(self, content: str) -> None: ...