docs(openapi): improve docs for session model

Merge google#5031

### Description

  This PR improves the generated OpenAPI/Swagger documentation for session
  management by adding clearer metadata and richer schema docs for the Session model
  (descriptions + examples).

  ### What changed

  - Session model docs: add field descriptions/examples for  so the API
    schema is self-explanatory.

  ### Why

  The OpenAPI output was correct but lacked enough context (field meanings
  and examples) for users reading Swagger UI or generating clients.

  ### Testing

  Documentation-only change (OpenAPI metadata / schema docs).
  - Unit tests: Not run.

Co-authored-by: Shangjie Chen <deanchen@google.com>
COPYBARA_INTEGRATE_REVIEW=google#5031 from akshay-kumar-bm:docs/openapi-session-endpoint-docs-update 04ea27e
PiperOrigin-RevId: 932788042

Author: 2 people

src/google/adk/sessions/session.py

@@ -29,26 +29,45 @@ class Session(BaseModel):
   """Represents a series of interactions between a user and agents."""
 
   model_config = ConfigDict(
-      extra='forbid',
+      extra="forbid",
       arbitrary_types_allowed=True,
       alias_generator=alias_generators.to_camel,
       populate_by_name=True,
   )
   """The pydantic model config."""
 
-  id: str
-  """The unique identifier of the session."""
-  app_name: str
-  """The name of the app."""
-  user_id: str
-  """The id of the user."""
-  state: dict[str, Any] = Field(default_factory=dict)
-  """The state of the session."""
-  events: list[Event] = Field(default_factory=list)
-  """The events of the session, e.g. user input, model response, function
-  call/response, etc."""
-  last_update_time: float = 0.0
-  """The last update time of the session."""
+  id: str = Field(
+      description="Unique identifier of the session.",
+      examples=["session-abc123"],
+  )
+  app_name: str = Field(
+      description="Application name that owns the session.",
+      examples=["hello_world"],
+  )
+  user_id: str = Field(
+      description="User ID that owns the session.",
+      examples=["user-123"],
+  )
+  state: dict[str, Any] = Field(
+      default_factory=dict,
+      description="Current persisted session state.",
+      examples=[{"locale": "en-US"}],
+  )
+  events: list[Event] = Field(
+      default_factory=list,
+      description=(
+          "Ordered event history for the session, including user, model, and"
+          " tool events (e.g. user input, model response, function"
+          " call/response)."
+      ),
+  )
+  last_update_time: float = Field(
+      default=0.0,
+      description=(
+          "Unix timestamp in seconds for the most recent session update."
+      ),
+      examples=[1_742_000_000.0],
+  )
 
   _storage_update_marker: str | None = PrivateAttr(default=None)
   """Internal storage revision marker used for stale-session detection."""