fix(runner): Yield buffered function_call/function_response events during live streaming

Bug: In live streaming mode, when function_call and function_response events
arrive during active transcription, they are correctly buffered but never
yielded to the caller. This causes callers to miss these events even though
they are saved to the session.

Fix: Add yield buffered_event after appending buffered events to the session
when transcription ends.

Testing:
- Added unit test: test_live_streaming_buffered_function_call_yielded_during_transcription
- Test verifies buffered events are yielded by:
  1. Simulating partial transcription (triggers buffering)
  2. Sending function_call during transcription (gets buffered)
  3. Ending transcription (should yield buffered events)
  4. Asserting both function_call and function_response are in yielded events

Test results:
- With fix: PASSED
- Without fix (yield commented out): FAILED with "Buffered function_call event was not yielded"
- Example event flow after fix:
EVENT: partial=True, input_transcription="Show me the weather"
EVENT: function_call=get_weather, args={'location': 'NYC'} <- Now yielded
EVENT: function_response=get_weather, response={...} <- Now yielded
EVENT: partial=False, input_transcription="Show me the weather for today"
PiperOrigin-RevId: 859158546

src/google/adk/runners.py

@@ -815,6 +815,7 @@ class Runner:
                   await self.session_service.append_event(
                       session=session, event=buffered_event
                   )
+                  yield buffered_event  # yield buffered events to caller
                 buffered_events = []
               else:
                 # non-transcription event or empty transcription event, for