fix(data_connect): fix UTF 8 characters decoding in data connect

packages/firebase_data_connect/firebase_data_connect/lib/src/network/rest_transport.dart

@@ -116,11 +116,12 @@ class RestTransport implements DataConnectTransport {
         headers: headers,
       );
       Map<String, dynamic> bodyJson =
-          jsonDecode(r.body) as Map<String, dynamic>;
+          jsonDecode(utf8.decode(r.bodyBytes)) as Map<String, dynamic>;
 
       if (r.statusCode != 200) {
-        String message =
-            bodyJson.containsKey('message') ? bodyJson['message']! : r.body;
+        String message = bodyJson.containsKey('message')
+            ? bodyJson['message']!
+            : utf8.decode(r.bodyBytes);
         throw DataConnectError(
           r.statusCode == 401
               ? DataConnectErrorCode.unauthorized

packages/firebase_data_connect/firebase_data_connect/test/src/network/rest_transport_test.dart

@@ -274,6 +274,44 @@ void main() {
       ).called(1);
     });
 
+    test(
+        'regression #17290 - invokeOperation should correctly decode UTF-8 response with international characters',
+        () async {
+      // Simulate a server response with Korean characters, where the
+      // Content-Type header does NOT include charset=utf-8 (which is
+      // what the Firebase emulator sends). Without explicit UTF-8
+      // decoding, the http package defaults to latin1, corrupting
+      // multi-byte characters.
+      const koreanJson =
+          '{"data": {"name": "\ud55c\uad6d\uc5b4 \ud14c\uc2a4\ud2b8"}}';
+      final mockResponse = http.Response.bytes(
+        utf8.encode(koreanJson),
+        200,
+        headers: {'content-type': 'application/json'},
+      );
+      when(
+        mockHttpClient.post(
+          any,
+          headers: anyNamed('headers'),
+          body: anyNamed('body'),
+        ),
+      ).thenAnswer((_) async => mockResponse);
+
+      final deserializer = (String data) => 'Deserialized Data';
+
+      final result = await transport.invokeOperation(
+        'testQuery',
+        'executeQuery',
+        deserializer,
+        null,
+        null,
+        null,
+      );
+
+      expect(result.data['data']['name'],
+          equals('\ud55c\uad6d\uc5b4 \ud14c\uc2a4\ud2b8'));
+    });
+
     test(
         'invokeOperation should handle missing auth and appCheck tokens gracefully',
         () async {