fix(firebaseai): keep LiveSession.receive() open after an unparseable message - #18752

Description

LiveSession.receive() is an async* generator over await for (final result in _messageController.stream). When the server sends a message that parseServerResponse doesn't know, for example the voiceActivity message from gemini-3.8-live, _listenToWebSocket adds a FirebaseAISdkException to the controller. await for then rethrows it inside the generator, so the listener gets onError and then onDone, even though the WebSocket and the controller are still open and the model keeps answering.

This switches the loop to yield* _messageController.stream. yield* forwards the error as an error event and keeps listening, so one unparseable frame becomes a single error event instead of ending the stream. The doc comment on receive() now says so.

This doesn't add a type for voiceActivity itself. I asked about that on the issue, since it would be new public API.

Related Issues

Fixes #18749

Checklist

• 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.
• I updated/added relevant documentation (doc comments with ///).
• The analyzer (melos run analyze) does not report any problems on my PR. (ran flutter analyze on firebase_ai)
• I read and followed the Flutter Style Guide.
• I signed the CLA.
• I am willing to follow-up on review comments in a timely manner.

Breaking Change

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

The new test in live_session_test.dart sends a valid message, a voiceActivity message and another valid message, then checks that the listener gets data, an error and data, with no onDone. It fails on main and passes with this change. The full firebase_ai suite passes (353 tests), and flutter analyze is clean for the package.