Fix tests with latest dependencies

[brianstrauch]

Summary

• keep the Google ADK streaming test compatible with supported versions before 2.7 while suppressing the new private-re-export diagnostic in ADK 2.7
• construct Google GenAI file-search mock results through Pydantic validation so deferred serializers are built before Temporal encodes activity results
• support both old and new Protobuf descriptor APIs without eagerly reading the label property removed in Protobuf 7
• fix the test-latest-deps failures observed in https://github.com/temporalio/sdk-python/actions/runs/33324609939/job/99296636937

Testing

With Python 3.13 and a temporary uv lock --upgrade / uv sync --all-extras environment:

• poe build-develop
• poe test -s tests/nexus/test_temporal_system_nexus.py tests/contrib/google_genai/test_gemini.py -k "test_system_nexus_proto_roundtrip or test_file_search_store_upload or test_full_integration_with_mock_client" (5 passed)
• poe lint