fix(google): finalize realtime generation when turn_complete is missing

[3DRX]

Problem

Some Gemini Live responses emit generation_complete but never emit turn_complete.
When that happens, RealtimeSession does not call _mark_current_generation_done(), so:

• generation channels can stay open longer than expected,
• downstream audio pipelines may not flush promptly,
• integrations that rely on end-of-segment signaling can truncate final playback.

What this changes

• Add TURN_COMPLETE_FALLBACK_SECONDS = 1.0.
• When generation_complete=True and turn_complete=False, schedule a fallback task that finalizes the active generation for that response_id.
• Cancel that fallback task on:
  • normal turn_complete completion,
  • generation replacement (_start_new_generation),
  • session shutdown (aclose),
  • explicit generation finalization (_mark_current_generation_done).

What this doesn't change

• Normal behavior is unchanged when turn_complete arrives.
• The fallback only applies to the non-standard/missing-event case.
• Response ID checks prevent finalizing a newer generation by mistake.

Tests

Added tests/test_plugin_google_realtime.py with coverage for:

• scheduling fallback on generation_complete without turn_complete,
• immediate finalization on turn_complete,
• fallback finalization path closing generation channels.

[CLAassistant]

All committers have signed the CLA.

[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

View in Devin Review to see 4 additional findings.