fix(graphql): guard query completer when stream emits multiple responses

QueryManager completed the same Completer on every Response from the link
stream. When the link emitted more than one event, the second completion
threw Bad state: Future already completed. Only complete the completer
while it is still pending, consistent with the onError path.

Reproduced when using a very short queryRequestTimeout (e.g. 1 ms) with
TimedInMemoryStore; that setup can emit multiple stream events and trigger
the crash. Example:

    return GraphQLClient(
      link: links,
      cache: GraphQLCache(
        store: TimedInMemoryStore(),
      ),
      queryRequestTimeout: const Duration(milliseconds: 1),
    );

Why merge: fixes a runtime crash for any setup where the link stream can
emit multiple values for a single query, and adds a regression test so the first emitted Response is kept when multiple emissions occur.

Author: tomwolx

packages/graphql/lib/src/core/query_manager.dart

@@ -282,13 +282,19 @@ class QueryManager {
       }
 
       // Listen for the first response or error
-      responseStream.listen(completer.complete,
-          onError: (Object error, StackTrace stackTrace) {
-        if (!completer.isCompleted) {
-          // We return the first error encountered
-          completer.completeError(error, stackTrace);
-        }
-      });
+      responseStream.listen(
+        (Response value) {
+          if (!completer.isCompleted) {
+            completer.complete(value);
+          }
+        },
+        onError: (Object error, StackTrace stackTrace) {
+          if (!completer.isCompleted) {
+            // We return the first error encountered
+            completer.completeError(error, stackTrace);
+          }
+        },
+      );
 
       // Await the response or error
       response = await completer.future;

packages/graphql/test/query_manager_test.dart

@@ -42,5 +42,31 @@ void main() {
       );
       client.queryManager.refetchQuery<dynamic>(observable.queryId);
     });
+
+    test('uses first response when link emits multiple events', () async {
+      const ping = 'ping';
+
+      when(link.request(any)).thenAnswer(
+        (_) => Stream.fromIterable(<Response>[
+          Response(
+            data: <String, dynamic>{ping: 1},
+            response: <String, dynamic>{},
+          ),
+          Response(
+            data: <String, dynamic>{ping: 2},
+            response: <String, dynamic>{},
+          ),
+        ]),
+      );
+
+      final result = await client.query(
+        QueryOptions(
+          document: parseString('{$ping}'),
+          fetchPolicy: FetchPolicy.networkOnly,
+        ),
+      );
+
+      expect(result.data![ping], 1);
+    });
   });
 }