test(buffer): pin exact ZREMRANGEBYRANK stop arg

Pinning -5_001 (= -(DEFAULT_EVENT_LIMIT) - 1) so the off-by-one
boundary is directly validated; expect.any(Number) would have passed
a wrong formula like -eventLimit.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Author: waleedlatif1

apps/sim/lib/copilot/request/session/buffer.test.ts

@@ -166,7 +166,7 @@ describe('mothership-stream-outbox', () => {
     expect(mockRedis.zremrangebyrank).toHaveBeenCalledWith(
       'mothership_stream:stream-1:events',
       0,
-      expect.any(Number)
+      -5_001
     )
   })