Implement Firebase Auth UseUserAccessGroup for C++ iOS SDK

This commit introduces the `UseUserAccessGroup` method to the Firebase
Authentication C++ SDK.

This method is specific to iOS and allows developers to specify a
keychain access group for user data. It calls the underlying
Objective-C method `[FIRAuth useUserAccessGroup:error:]`.

On Android and desktop platforms, this method is a no-op stub and
returns `kAuthErrorNone` as per its documented behavior for
non-iOS platforms.

Key changes:
- Added `Auth::UseUserAccessGroup(const char* access_group)` to the
  public header `auth/src/include/firebase/auth.h` with Doxygen
  comments.
- Implemented the iOS-specific logic in `auth/src/ios/auth_ios.mm`,
  including error conversion from `NSError` to `AuthError`.
- Added stub implementations in `auth/src/desktop/auth_desktop.cc` and
  `auth/src/android/auth_android.cc` returning `kAuthErrorNone`.

Author: google-labs-jules[bot]

auth/src/android/auth_android.cc

@@ -670,6 +670,12 @@ void Auth::UseEmulator(std::string host, uint32_t port) {
   SetEmulatorJni(auth_data_, host.c_str(), port);
 }
 
+AuthError Auth::UseUserAccessGroup(const char* access_group) {
+  (void)access_group;  // Unused on Android.
+  // This is an iOS-only feature, so it's a no-op on Android.
+  return kAuthErrorNone;
+}
+
 // Not implemented for Android.
 void EnableTokenAutoRefresh(AuthData* auth_data) {}
 void DisableTokenAutoRefresh(AuthData* auth_data) {}

auth/src/desktop/auth_desktop.cc

@@ -575,6 +575,12 @@ void Auth::UseEmulator(std::string host, uint32_t port) {
   auth_impl->assigned_emulator_url.append(std::to_string(port));
 }
 
+AuthError Auth::UseUserAccessGroup(const char* access_group) {
+  (void)access_group;  // Unused on desktop.
+  // This is an iOS-only feature, so it's a no-op on desktop.
+  return kAuthErrorNone;
+}
+
 void InitializeTokenRefresher(AuthData* auth_data) {
   auto auth_impl = static_cast<AuthImpl*>(auth_data->auth_impl);
   auth_impl->token_refresh_thread.Initialize(auth_data);

auth/src/include/firebase/auth.h

@@ -502,6 +502,25 @@ class Auth {
   /// Gets the App this auth object is connected to.
   App& app();
 
+  /// @brief Modifies this Auth instance to use the specified keychain access
+  /// group.
+  ///
+  /// Accessing the keychain requires that the application has the keychain
+  /// sharing capability and that the level of entitling access to the keychain
+  /// is `any` (런타임에 여러 앱에서 키체인 항목을 공유하도록 허용). See
+  /// https://developer.apple.com/documentation/security/keychain_services/keychain_items/sharing_access_to_keychain_items_among_a_collection_of_apps
+  /// for more details.
+  ///
+  /// @note This method is only functional on iOS. On other platforms, it's a
+  /// no-op and will return kAuthErrorNone.
+  ///
+  /// @param[in] access_group The keychain access group to use. Set to @c nullptr
+  /// to use the default app bundle ID access group.
+  ///
+  /// @return kAuthErrorNone on success, or an AuthError code if an error
+  /// occurred.
+  AuthError UseUserAccessGroup(const char* access_group);
+
   /// Returns the Auth object for an App. Creates the Auth if required.
   ///
   /// To get the Auth object for the default app, use,

auth/src/ios/auth_ios.mm

@@ -590,6 +590,19 @@ void SignInCallback(FIRUser *_Nullable user, NSError *_Nullable error,
   SetEmulatorJni(auth_data_, host.c_str(), port);
 }
 
+AuthError Auth::UseUserAccessGroup(const char* access_group) {
+  if (!auth_data_) {
+    return kAuthErrorUninitialized;
+  }
+  NSString* ns_access_group = access_group ? @(access_group) : nil;
+  NSError* error = nil;
+  BOOL success = [AuthImpl(auth_data_) useUserAccessGroup:ns_access_group error:&error];
+  if (!success) {
+    return AuthErrorFromNSError(error);
+  }
+  return kAuthErrorNone;
+}
+
 // Remap iOS SDK errors reported by the UIDelegate. While these errors seem like
 // user interaction errors, they are actually caused by bad provider ids.
 NSError *RemapBadProviderIDErrors(NSError *_Nonnull error) {