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(() {