refactor: streamline agent controller and remove legacy resumable handling

- Updated the AgentController to route all requests to ResumableAgentController, simplifying the logic.
- Deprecated the legacy non-resumable path, providing a clear migration path for future use.
- Adjusted setHeaders middleware to remove unnecessary checks for resumable mode.
- Cleaned up the useResumableSSE hook to eliminate redundant query parameters, enhancing clarity and performance.

api/server/middleware/setHeaders.js

@@ -1,9 +1,4 @@
 function setHeaders(req, res, next) {
-  // Skip SSE headers for resumable mode - it returns JSON first, then client subscribes separately
-  if (req.query.resumable === 'true') {
-    return next();
-  }
-
   res.writeHead(200, {
     Connection: 'keep-alive',
     'Content-Type': 'text/event-stream',