fix: make InvocationEvent.content optional so the Web UI can save eval cases

InvocationEvent.content was typed Optional[Content] but had no default. Under
Pydantic v2 an Optional field without a default is still required, so saving an
eval case whose event carries no content (as the Web UI does) failed model
validation and the save PUT returned HTTP 422. Default content to None so an
omitted or empty content is accepted.

Close google#6336

Co-authored-by: George Weale <gweale@google.com>
PiperOrigin-RevId: 951053413

Author: GWeale

src/google/adk/evaluation/eval_case.py

@@ -64,7 +64,7 @@ class InvocationEvent(EvalBaseModel):
   author: str
   """The name of the agent that authored/owned this event."""
 
-  content: Optional[genai_types.Content]
+  content: Optional[genai_types.Content] = None
   """The content of the event."""
 
 

tests/unittests/evaluation/test_eval_case.py

@@ -54,6 +54,14 @@ def test_eval_models_preserve_extra_metadata():
   assert dumped['session_input']['source'] == 'nightly'
 
 
+def test_invocation_event_content_defaults_to_none():
+  """An InvocationEvent can be built and round-tripped without content."""
+  event = InvocationEvent(author='agent')
+
+  assert event.content is None
+  assert InvocationEvent.model_validate(event.model_dump()).content is None
+
+
 def test_get_all_tool_calls_with_none_input():
   """Tests that an empty list is returned when intermediate_data is None."""
   assert get_all_tool_calls(None) == []