fix: catch stop() already called on concurrent 429s

Multiple concurrent requests can receive 429 simultaneously, causing
stop() to be called on an already-stopped limiter. Add .catch() to
prevent unhandled rejection.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

open-sse/services/rateLimitManager.ts

@@ -352,7 +352,7 @@ export function updateFromHeaders(provider, connectionId, headers, status, model
     // After stop, delete from Map so getLimiter() creates a fresh instance.
     limiter.stop({ dropWaitingJobs: true }).then(() => {
       limiters.delete(limiterKey);
-    });
+    }).catch(() => {});
     return;
   }