fix(google): support gemini-3.1-flash-live-preview realtime API for g…

[acharan-tech-200037]

This pr resolves #5260

Adds support for gemini-3.1-flash-live-preview in the Google Realtime plugin.

Problem

The gemini-3.1-flash-live-preview model (released March 26, 2026) rejects all send_client_content WebSocket calls with a 1007 error (Request contains an invalid argument). This causes:

• generate_reply() to timeout waiting for generation_created event
• Initial chat context delivery to fail on connection
• Mid-session updates via update_instructions() / update_chat_ctx() to fail

The model only accepts send_realtime_input for all content delivery.

How it was confirmed

A minimal test script was used to isolate the issue:

• send_client_content → ❌ 1007 error (even with minimal config)
• send_realtime_input with text → ✅ Works perfectly

Changes

File: livekit-plugins-google/livekit/plugins/google/realtime/realtime_api.py

1. Added gemini-3.1-flash-live-preview to KNOWN_GEMINI_API_MODELS
2. Added REALTIME_INPUT_ONLY_MODELS constant to detect models that reject send_client_content
3. generate_reply() — Routes instructions as text via LiveClientRealtimeInput instead of LiveClientContent
4. _main_task() — Converts initial chat context to send_realtime_input with [role]: text prefixes
5. _send_task() — Catch-all: converts any LiveClientContent to send_realtime_input with role prefixes (covers update_instructions and update_chat_ctx flows)

All changes are backward-compatible — older models continue using send_client_content as before.

[CLAassistant]

All committers have signed the CLA.

[acharan-tech-200037]

@livekit/maintainers This PR is ready for review!

Summary: Adds support for Gemini 3.1 Flash Live Preview by switching from send_client_content to send_realtime_input for models that don't support the former.

Changes:

• Added REALTIME_INPUT_ONLY_MODELS constant
• Patched generate_reply, _send_task, and _main_task to use send_realtime_input
• Fixed update_instructions to reconnect for these models
• Included Python 3.9 compatibility fix for cambia plugin (required for CI)

Testing:

• ✅ Gemini 3.1 works
• ✅ Gemini 2.5 unaffected
• ✅ All CI checks passed

Ready for review when you have time. Thanks!

[acharan-tech-200037]

@theomonnom @tinalenguyen can you please review this pr.
This pr solves the issue #5260 along with failing on update_chat_ctx and update_instructions by gemini 3.1 flash live preview.

[acharan-tech-200037]

Closing this in favor of #5251 by @Hormold which takes a cleaner
approach to the same problem.

Key differences:

• Uses history_config.initial_history_in_client_content=True for
  proper initial seeding instead of converting turns to plain text
• Drops mid-session LiveClientContent cleanly instead of converting
  to degraded text blobs
• Adds empty server_content guard to prevent dropped audio on 3.1
• Adds reconnect_on_update opt-in parameter for developer control

Tested and Hormold's approach is architecturally correct.
Recommend merging #5251.