feat(fcm): Migrate topic management to FCM v1 REST API

[lahirumaramba]

Migrates topic subscription and management from the deprecated Instance ID (IID) service to the FCM v1 REST API. The new implementation calls the FCM v1 topicSubscriptions endpoints for each registration token concurrently (bounded by a semaphore) and handles idempotent subscription scenarios (HTTP 409 / ALREADY_EXISTS). Deprecates the legacy IID topic methods.

[gemini-code-assist]

Code Review

This pull request introduces new non-legacy SubscribeToTopicAsync and UnsubscribeFromTopicAsync methods to FirebaseMessaging and FirebaseMessagingClient that utilize the new v1 FCM API, while deprecating the old Instance ID API-based methods. It also adds validation, concurrent request handling using SemaphoreSlim, updated error mapping, and comprehensive unit tests. The review feedback highlights three key areas for improvement: explicitly catching and rethrowing OperationCanceledException to prevent swallowing cancellations in the generic catch block, properly disposing of the SemaphoreSlim instance to avoid resource leaks, and using RegexOptions.Compiled on the static topic name Regex to optimize performance.

[gemini-code-assist]

The generic catch (Exception) block will catch OperationCanceledException (or TaskCanceledException) when the cancellationToken is triggered, converting the cancellation into a failed TopicResult with "UNKNOWN_ERROR". This prevents proper cancellation propagation. Catch and rethrow OperationCanceledException explicitly before catching general exceptions.

            catch (FirebaseMessagingException e)
            {
                if (isSubscribe && IsAlreadyExists(e))
                {
                    return TopicResult.Success(index);
                }

                var reason = ExtractReason(e);
                return TopicResult.Failure(index, reason);
            }
            catch (OperationCanceledException)
            {
                throw;
            }
            catch (Exception)
            {
                return TopicResult.Failure(index, "UNKNOWN_ERROR");
            }

[gemini-code-assist]

The SemaphoreSlim instance is created per request but is never disposed. Since SemaphoreSlim implements IDisposable and can allocate system wait handles, it should be disposed to prevent potential resource leaks. Wrap its creation and usage in a using block.

            using (var semaphore = new SemaphoreSlim(Math.Min(registrationTokens.Count, 100)))
            {
                var tasks = new List<Task<TopicResult>>(registrationTokens.Count);

                for (int i = 0; i < registrationTokens.Count; i++)
                {
                    var index = i;
                    var token = registrationTokens[i];
                    tasks.Add(this.SendSingleTopicRequestAsync(
                        token, encodedTopic, isSubscribe, index, semaphore, cancellationToken));
                }

                var results = await Task.WhenAll(tasks).ConfigureAwait(false);
                var successCount = 0;
                var errors = new List<ErrorInfo>();

                foreach (var result in results)
                {
                    if (result.IsSuccess)
                    {
                        successCount++;
                    }
                    else
                    {
                        errors.Add(new ErrorInfo(result.Index, result.Reason));
                    }
                }

                return new TopicManagementResponse(successCount, errors);
            }

[gemini-code-assist]

For static Regex instances that are used repeatedly, it is highly recommended to specify RegexOptions.Compiled to improve performance by compiling the regular expression to intermediate language instead of interpreting it at runtime.

        private static readonly System.Text.RegularExpressions.Regex TopicNamePattern =
            new System.Text.RegularExpressions.Regex("^(/topics/)?(private/)?[a-zA-Z0-9-_.~%]+$", System.Text.RegularExpressions.RegexOptions.Compiled);