fix(firebaseai): keep LiveSession.receive() open on voiceActivity frames

[SelaseKay]

Description

gemini-3.8-live sends a voiceActivity frame when the user starts or stops speaking. firebase_ai did not model that message, so parsing threw FirebaseAISdkException. LiveSession.receive() is an await for over the broadcast controller, so that error closed the Dart stream while the WebSocket stayed open. Apps that treat onDone as a closed session lost the rest of the spoken turn.

This parses voiceActivity into LiveServerVoiceActivity (type and audioOffset). An unrecognized activity type still yields the message, with type left null. A top-level key the SDK does not recognize is logged and skipped, so receive() stays open for the next frame. A server error payload or invalid JSON still ends the stream.

An exhaustive switch on LiveServerMessage needs a new case, the same way GoingAwayNotice did.

Related Issues

Fixes #18749

Checklist

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• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• My PR includes unit or integration tests for all changed/updated/fixed behaviors (See Contributor Guide).
• All existing and new tests are passing.
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Breaking Change

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• Yes, this is a breaking change.
• No, this is not a breaking change.

Test plan

• flutter test test/live_test.dart test/live_session_test.dart in packages/firebase_ai/firebase_ai.
• On device, gemini-3.8-live with microphone audio keeps LiveSession.receive() open after ACTIVITY_START, and later audio or turnComplete frames still arrive.
• An unrecognized live frame is logged and does not close receive().

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