Stabilize Guardian WebSocket tests (#41191)

## Why

Guardian tests could continue after a WebSocket handshake but before the
connection entered the idle pool. The shared request waiter could also miss a
notification delivered between checking the request log and registering its
wait.

## What changed

- Wait for Guardian prewarming to populate the idle connection pool instead of
  treating completed handshakes as sufficient.
- Use two initial Guardian connections in tests while retaining eight in
  non-test builds.
- Register for request-log notifications before inspecting the log to avoid a
  lost wakeup.

GitOrigin-RevId: 88f6a22c5401e03c24e6e114f2563fc641bfef80

codex-rs/core/tests/common/responses.rs

@@ -604,6 +604,7 @@ impl WebSocketTestServer {
         request_index: usize,
     ) -> WebSocketRequest {
         loop {
+            let notified = self.request_log_updated.notified();
             if let Some(request) = self
                 .connections
                 .lock()
@@ -614,7 +615,7 @@ impl WebSocketTestServer {
             {
                 return request;
             }
-            self.request_log_updated.notified().await;
+            notified.await;
         }
     }