fix(models): Prevent grounding metadata loss in Gemini 3.1

In Gemini 3.1 Live, grounding metadata is often emitted early alongside partial chunks or immediately after tool execution, causing the runner to discard it because it is associated with partial=True events.

- Retain pending grounding metadata during the turn in GeminiLlmConnection.receive and yield it on the final non-partial event.
- Add test_receive_grounding_metadata_pending unit test to verify the fix.

Change-Id: I4d8ab1f231a3fed63c4375cbfcd8831d6535721e