fix(consultations): open chat via consultation chat_id

Use chat_id from the consultations payload for direct navigation and keep
the chat lookup as a fallback for legacy rows without chat_id.

- validate chat id in ChatRepository and make it nullable
- guard repeated taps while opening chat
- handle nullable chat id in ProfileCubit and EmptyChatScreen

related to #1106

lib/features/profile/presentation/bloc/profile_cubit/profile_cubit.dart

@@ -141,13 +141,13 @@ class ProfileCubit extends Cubit<ProfileState> {
 
   Future<String?> getChatRoomIdBySlug({required String slug}) async {
     try {
-      final chatIdOrError = await chatRepository.getChatByUserSlug(slug);
-      final userProfile = await profileRepository.getProfileBySlug(slug);
-      if (chatIdOrError == '404') {
+      final chatId = await chatRepository.getChatByUserSlug(slug);
+      if (chatId == null || chatId.isEmpty) {
+        final userProfile = await profileRepository.getProfileBySlug(slug);
         AppRouter.navigatorKey.currentContext?.pushNamed('emptyChatScreen', extra: userProfile);
       } else {
-        AppRouter.navigatorKey.currentContext?.pushNamed('chatScreen', extra: {'uuid': chatIdOrError});
-        return chatIdOrError;
+        AppRouter.navigatorKey.currentContext?.pushNamed('chatScreen', extra: {'uuid': chatId});
+        return chatId;
       }
     } catch (e, st) {
       await reportFrom(e, st, this);