Validate stale chat requests, show info instead of auto-dismiss

- Add validateIncomingRequest() — checks session status with backend
- Home screen validates before showing sheet (on resume + listener)
- IncomingRequestSheet shows "cancelled/accepted by other" message
  instead of silently dismissing when request becomes stale

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

mitra_app/lib/core/chat/chat_request_notifier.dart

@@ -130,6 +130,22 @@ class ChatRequest extends _$ChatRequest {
     }
   }
 
+  /// Check if the current incoming request is still valid (pending_acceptance).
+  /// If stale, reset to listening state.
+  Future<void> validateIncomingRequest() async {
+    if (state is! ChatRequestIncomingData) return;
+    final sessionId = (state as ChatRequestIncomingData).sessionId;
+    try {
+      final response = await _apiClient.get('/api/shared/chat/$sessionId/info');
+      final status = response['data']?['status'] as String?;
+      if (status != 'pending_acceptance') {
+        state = const ChatRequestListeningData();
+      }
+    } catch (_) {
+      state = const ChatRequestListeningData();
+    }
+  }
+
   Future<void> accept(String sessionId) async {
     state = const ChatRequestAcceptingData();
     try {