chore: Clarify the behavior of Event.invocation_id

PiperOrigin-RevId: 774827874

Author: DeanChensj

src/google/adk/events/event.py

@@ -34,9 +34,10 @@ class Event(LlmResponse):
  taken by the agents like function calls, etc.
 
  Attributes:
- invocation_id: The invocation ID of the event.
- author: "user" or the name of the agent, indicating who appended the event
- to the session.
+ invocation_id: Required. The invocation ID of the event. Should be non-empty
+ before appending to a session.
+ author: Required. "user" or the name of the agent, indicating who appended
+ the event to the session.
  actions: The actions taken by the agent.
  long_running_tool_ids: The ids of the long running function calls.
  branch: The branch of the event.
@@ -55,9 +56,8 @@ class Event(LlmResponse):
  )
  """The pydantic model config."""
 
- # TODO: revert to be required after spark migration
  invocation_id: str = ''
- """The invocation ID of the event."""
+ """The invocation ID of the event. Should be non-empty before appending to a session."""
  author: str
  """'user' or the name of the agent, indicating who appended the event to the
  session."""