Move Mistral message fixups into MistralLLMAdapter

Mistral imposes three conversation-history quirks on top of the
OpenAI-compatible wire format: tool messages must be followed by an
assistant message; non-initial system messages are rejected; trailing
assistant messages require `prefix=True`. These rules were applied
inline in `MistralLLMService.build_chat_completion_params`, which is the
wrong layer — every other provider with OpenAI-compatible-but-quirky
shape (Perplexity, etc.) owns its transformations in a
`BaseLLMAdapter` subclass that runs during `get_llm_invocation_params`.

Create `MistralLLMAdapter(OpenAILLMAdapter)` on the Perplexity template
and wire it in via the existing `adapter_class` dispatch. The service
now only handles Mistral-specific request-level mapping (`random_seed`
in place of `seed`), and the message shape concerns live with other
provider format logic.

No behavior change. The transform function casts to `list[dict[str,
Any]]` internally because mutating `role` and attaching Mistral's
non-standard `prefix` field both step outside OpenAI's TypedDict
contract; the cast at the return boundary encodes that we're emitting
Mistral's extended schema, not OpenAI's.

pyrightconfig.json

@@ -2,14 +2,8 @@
   "typeCheckingMode": "basic",
   "pythonVersion": "3.11",
   "pythonPlatform": "All",
-  "include": [
-    "scripts",
-    "src/pipecat"
-  ],
-  "exclude": [
-    "**/*_pb2.py",
-    "**/__pycache__"
-  ],
+  "include": ["scripts", "src/pipecat"],
+  "exclude": ["**/*_pb2.py", "**/__pycache__"],
   "ignore": [
     "tests",
     "src/pipecat/adapters/services/anthropic_adapter.py",
@@ -82,7 +76,6 @@
     "src/pipecat/services/llm_service.py",
     "src/pipecat/services/lmnt/tts.py",
     "src/pipecat/services/mem0/memory.py",
-    "src/pipecat/services/mistral/llm.py",
     "src/pipecat/services/mistral/stt.py",
     "src/pipecat/services/mistral/tts.py",
     "src/pipecat/services/moondream/vision.py",