fix(ios-audio): keep session active when already-in-channel

The 'already in channel' server response (code 0x0302) is treated as a
successful join by _onJoinChannel: the user stays in the channel and
local state is updated to reflect the joined target. But the underlying
voiceJoin call still raises BridgeError_ServerRejected, which the
joinVoiceChannelWithIosAudioSession helper used to interpret as a join
failure and deactivate the iOS audio session. Result: the UI shows the
user as joined while the audio session is dead and capture/playback
remain silent.

Add an isJoinSuccess predicate to the ordering helper. When the
predicate matches, the helper rethrows (so the caller can still run its
success-on-already-joined branch) without deactivating the session.
Wire _onJoinChannel to pass _isAlreadyInChannel as the predicate so the
0x0302 path keeps the session active.

Adds two regression tests covering the success-on-rethrow and the
predicate-false-still-deactivates paths.

apps/chanora_flutter/lib/main.dart

@@ -1308,6 +1308,11 @@ class _BetaHomeState extends State<_BetaHome> with WidgetsBindingObserver {
         voiceJoin: rust.voiceJoin,
         activateIosAudioSession: iosAudioSessionController.activate,
         deactivateIosAudioSession: iosAudioSessionController.deactivate,
+        // Server says we are already in the target channel: the user is
+        // still joined to a voice channel, so the iOS audio session must
+        // stay active. The catch below converts this rethrow into the
+        // success-on-already-joined branch.
+        isJoinSuccess: _isAlreadyInChannel,
       );
       if (!mounted) return;
       setState(() {

apps/chanora_flutter/lib/services/voice_join_ordering.dart

@@ -6,17 +6,30 @@ typedef VoiceJoinCallback = Future<void> Function({
 typedef IosVoiceSessionActivation = Future<void> Function();
 typedef IosVoiceSessionDeactivation = Future<void> Function();
 
+/// Predicate used to recognise `voiceJoin` errors that the caller treats as a
+/// successful join outcome (e.g. the server replied "already in channel").
+///
+/// When this returns `true` for a thrown error, the iOS audio session is kept
+/// active because the user is still considered joined to the channel. The
+/// error is still rethrown so the caller can run its success-on-already-joined
+/// branch and update local state.
+typedef VoiceJoinSuccessPredicate = bool Function(Object error);
+
 Future<void> joinVoiceChannelWithIosAudioSession({
   required BigInt channelId,
   required String password,
   required VoiceJoinCallback voiceJoin,
   required IosVoiceSessionActivation activateIosAudioSession,
   required IosVoiceSessionDeactivation deactivateIosAudioSession,
+  VoiceJoinSuccessPredicate? isJoinSuccess,
 }) async {
   await activateIosAudioSession();
   try {
     await voiceJoin(channelId: channelId, password: password);
-  } catch (_) {
+  } catch (e) {
+    if (isJoinSuccess != null && isJoinSuccess(e)) {
+      rethrow;
+    }
     await deactivateIosAudioSession();
     rethrow;
   }