fix: stop exceptions from callbacks unwinding through WebRTC

[devopvoid]

Builds on #334 (now merged), which moved the ReportUncaughtException helper to WebRTCUtils.

Problem

The callbacks WebRTC invokes ended with ExceptionCheck(env), which turns a pending Java exception into a C++ exception. When the application's observer threw, or converting a callback's arguments failed, that exception unwound through WebRTC's frames. WebRTC is built without unwinding support, so this leaves its locks held, which shows up later as a silent hang (the same failure as the earlier LogSink deadlock), or ends the process, depending on the platform.

Fix

Every callback that WebRTC invokes now catches its own C++ exceptions and turns them into Java ones (ThrowCxxJavaException), then ends with the new ReportPendingException(env), which clears the pending exception and hands it to the calling thread's uncaught exception handler, as the JVM does for an exception escaping a Java thread. Converted:

• PeerConnectionObserver (all callbacks), CreateSessionDescriptionObserver, SetSessionDescriptionObserver
• RTCDataChannelObserver, RTCDtlsTransportObserver, RTCDtmfSenderObserver, RTCStatsCollectorCallback
• AudioTrackSink, VideoTrackSink, MediaStreamTrackObserver
• AudioTransportSink (the audio device's recorded-data sink), DesktopCaptureCallback

The observers from #334 and the data channel send observer, which already reported exceptions this way, now share the helper instead of carrying their own copy.

Converters that throw are unchanged: they run inside JNI calls or callbacks that now catch.

Also fixed in these callbacks

• AudioTrackSink and AudioTransportSink hold their byte array in a JavaLocalRef, so it is freed when the callback throws (on a native thread a leaked local reference is never reclaimed), and pass jint values to the Java callback instead of size_t through varargs.
• MediaStreamTrackObserver released the Java track's local reference on every mute or end event instead of deleting it.
• CustomVideoSource.pushFrame and pushFrameTimestamped let a C++ exception from converting the frame leave the JNI call; they now throw it as a Java exception.

Tests

ObserverExceptionTests makes the stats callback, the create-offer observer, the peer connection observer (onIceCandidate, onConnectionChange) and a data channel observer throw on every call, and checks that each exception reaches the uncaught exception handler, that WebRTC keeps working (the next stats and offer arrive, the peers connect, every message is delivered), and that close() returns within 10 seconds.

The full webrtc suite passes on Windows (253 run, 0 failures, 2 skipped), also under -Pjni-check with no JNI errors.