[AI] Implement RealtimeInputConfig and Manual Activity Signals for Live API

[milaGGL]

Add support for RealtimeInputConfig to configure voice activity detection and manual activity signals in the Live API.

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[github-actions]

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[milaGGL]

/gemini review

[gemini-code-assist]

Code Review

This pull request introduces comprehensive support for realtime input configuration in the Live API, specifically adding LiveRealtimeInputConfig and LiveActivityDetection. These additions allow for fine-grained control over how the model detects user activity, handles interruptions, and defines turn coverage. The PR includes new public methods sendStartActivityRealtime and sendStopActivityRealtime for manual activity signaling, along with corresponding updates to the Java-friendly LiveSessionFutures and internal serialization logic. Comprehensive unit and instrumentation tests have been added to verify the new configuration options. Review feedback focuses on minor documentation refinements to ensure consistent KDoc linking syntax and improved readability in the LiveSession class.