chore: Updates model_config docstring to explicitly mentioning pydantic.

This is to disambiguate between pydantic model and LLM.

PiperOrigin-RevId: 754981617

src/google/adk/sessions/session.py

@@ -38,6 +38,7 @@ class Session(BaseModel):
       extra='forbid',
       arbitrary_types_allowed=True,
   )
+  """The pydantic model config."""
 
   id: str
   """The unique identifier of the session."""