🚦 refactor: Concurrent Request Limiter for Resumable Streams (#11167)

* feat: Implement concurrent request handling in ResumableAgentController

- Introduced a new concurrency management system by adding `checkAndIncrementPendingRequest` and `decrementPendingRequest` functions to manage user request limits.
- Replaced the previous `concurrentLimiter` middleware with a more integrated approach directly within the `ResumableAgentController`.
- Enhanced violation logging and request denial for users exceeding their concurrent request limits.
- Removed the obsolete `concurrentLimiter` middleware file and updated related imports across the codebase.

* refactor: Simplify error handling in ResumableAgentController and enhance SSE error management

- Removed the `denyRequest` middleware and replaced it with a direct response for concurrent request violations in the ResumableAgentController.
- Improved error handling in the `useResumableSSE` hook to differentiate between network errors and other error types, ensuring more informative error responses are sent to the error handler.

* test: Enhance MCP server configuration tests with new mocks and improved logging

- Added mocks for MCP server registry and manager in `index.spec.js` to facilitate testing of server configurations.
- Updated debug logging in `initializeMCPs.spec.js` to simplify messages regarding server configurations, improving clarity in test outputs.

* refactor: Enhance concurrency management in request handling

- Updated `checkAndIncrementPendingRequest` and `decrementPendingRequest` functions to utilize Redis for atomic request counting, improving concurrency control.
- Added error handling for Redis operations to ensure requests can proceed even during Redis failures.
- Streamlined cache key generation for both Redis and in-memory fallback, enhancing clarity and performance in managing pending requests.
- Improved comments and documentation for better understanding of the concurrency logic and its implications.

* refactor: Improve atomicity in Redis operations for pending request management

- Updated `checkAndIncrementPendingRequest` to utilize Redis pipelines for atomic INCR and EXPIRE operations, enhancing concurrency control and preventing edge cases.
- Added error handling for pipeline execution failures to ensure robust request management.
- Improved comments for clarity on the concurrency logic and its implications.

client/src/hooks/SSE/useResumableSSE.ts

@@ -532,9 +532,20 @@ export default function useResumableSSE(
         }
       }
 
-      // All retries failed or non-network error
       console.error('[ResumableSSE] Error starting generation:', lastError);
-      errorHandler({ data: undefined, submission: currentSubmission as EventSubmission });
+
+      const axiosError = lastError as { response?: { data?: Record<string, unknown> } };
+      const errorData = axiosError?.response?.data;
+      if (errorData) {
+        errorHandler({
+          data: { text: JSON.stringify(errorData) } as unknown as Parameters<
+            typeof errorHandler
+          >[0]['data'],
+          submission: currentSubmission as EventSubmission,
+        });
+      } else {
+        errorHandler({ data: undefined, submission: currentSubmission as EventSubmission });
+      }
       setIsSubmitting(false);
       return null;
     },