Address review comments for UseUserAccessGroup

- Moved stub implementations from auth_stub.cc to auth_desktop.cc and auth_android.cc.
- Removed auth_stub.cc.
- Removed unnecessary forward declaration for UseUserAccessGroupInternal from auth.cc.
- Added a basic integration test for UseUserAccessGroup to ensure it doesn't crash on any platform.

Author: google-labs-jules[bot]

auth/integration_test/src/integration_test.cc

@@ -1513,4 +1513,47 @@ TEST_F(FirebaseAuthTest, TestLinkFederatedProviderBadProviderIdFails) {
 
 #endif  // defined(ENABLE_OAUTH_TESTS)
 
+TEST_F(FirebaseAuthTest, TestUseUserAccessGroup) {
+  // This test simply calls the UseUserAccessGroup method to ensure it doesn't
+  // crash. It doesn't verify the underlying keychain behavior.
+  // On non-iOS platforms, this is a no-op and should return kAuthErrorNone.
+  // On iOS, it will call the underlying OS method.
+
+  LogDebug("Calling UseUserAccessGroup with a test group ID.");
+  firebase::auth::AuthError error =
+      auth_->UseUserAccessGroup("com.firebase.test.accessgroup");
+
+#if TARGET_OS_IPHONE
+  // On iOS, the actual error code depends on keychain access and provisioning.
+  // For this test, we just ensure it doesn't crash.
+  // A specific error like kAuthErrorMissingIOSBundleID might occur if the
+  // environment isn't set up for keychain sharing, or kAuthErrorNone on
+  // success. For a simple "does not crash" test, we don't strictly check
+  // the error code, but kAuthErrorNone is the ideal outcome if keychain is
+  // usable.
+  LogDebug("UseUserAccessGroup returned %d on iOS.", error);
+#else
+  // On other platforms, it should be a no-op.
+  EXPECT_EQ(error, firebase::auth::kAuthErrorNone);
+#endif
+
+  LogDebug("Calling UseUserAccessGroup with nullptr to clear the group.");
+  error = auth_->UseUserAccessGroup(nullptr);
+
+#if TARGET_OS_IPHONE
+  LogDebug("UseUserAccessGroup(nullptr) returned %d on iOS.", error);
+#else
+  EXPECT_EQ(error, firebase::auth::kAuthErrorNone);
+#endif
+
+  LogDebug("Calling UseUserAccessGroup with empty string to clear the group.");
+  error = auth_->UseUserAccessGroup("");
+
+#if TARGET_OS_IPHONE
+  LogDebug("UseUserAccessGroup(\"\") returned %d on iOS.", error);
+#else
+  EXPECT_EQ(error, firebase::auth::kAuthErrorNone);
+#endif
+}
+
 }  // namespace firebase_testapp_automated

auth/src/auth.cc

@@ -373,12 +373,6 @@ AUTH_RESULT_FN(Auth, SignInWithEmailAndPassword, AuthResult)
 
 AUTH_RESULT_FN(Auth, CreateUserWithEmailAndPassword, AuthResult)
 
-// Platform-specific implementation of UseUserAccessGroup.
-// This is defined in auth_ios.mm for iOS, and auth_stub.cc for other
-// platforms.
-AuthError UseUserAccessGroupInternal(AuthData* auth_data,
-                                     const char* group_id);
-
 AuthError Auth::UseUserAccessGroup(const char* group_id) {
   if (!auth_data_) {
     return kAuthErrorUninitialized;

auth/src/desktop/auth_desktop.cc

@@ -768,5 +768,14 @@ void IdTokenRefreshThread::DisableAuthRefresh() {
   ref_count_--;
 }
 
+// Stub implementation for UseUserAccessGroupInternal on desktop.
+AuthError UseUserAccessGroupInternal(AuthData* auth_data,
+                                     const char* group_id) {
+  // This function is a no-op on desktop platforms.
+  (void)auth_data;
+  (void)group_id;
+  return kAuthErrorNone;
+}
+
 }  // namespace auth
 }  // namespace firebase