fix(voice-call): Improve async handling and state management

Refactor voice call service to handle asynchronous operations more
precisely. Update method signatures to be async, use unawaited for
non-blocking calls, and ensure proper state reset between sessions.
Improve error handling and resource management for voice input and
text-to-speech services.

lib/features/chat/services/text_to_speech_service.dart

@@ -149,6 +149,9 @@ class TextToSpeechService {
   Future<void> dispose() async {
     await _eventSubscription?.cancel();
     _eventSubscription = null;
+
+    // Reset the singleton state for next session
+    await TtsManager.instance.reset();
   }
 
   /// Updates TTS settings.