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

test/features/chat/repositories/chat_repository_test.dart

@@ -526,11 +526,11 @@ void main() {
       },
     );
 
-    test('getChatByUserSlug - возвращает 404 при отсутствии чата', () async {
+    test('getChatByUserSlug - возвращает null, если в ответе нет id', () async {
       const slug = 'user-slug';
       final response = HttpResponse(
         {},
-        Response(requestOptions: RequestOptions(path: ''), statusCode: 404),
+        Response(requestOptions: RequestOptions(path: ''), statusCode: 200),
       );
 
       when(
@@ -539,7 +539,7 @@ void main() {
 
       final result = await chatRepository.getChatByUserSlug(slug);
 
-      expect(result, '404');
+      expect(result, isNull);
       verify(mockChatService.getChatByUserSlug(slug: slug)).called(1);
     });