Defer Noise environment connections until registration (#33166)

## What changed

- Replace pending WebSocket URL registration with deferred Noise environment
  registration that gates connection attempts on an explicit readiness signal.
- Reuse the Noise rendezvous transport after readiness so reconnects request a
  fresh connection bundle.
- Preserve terminal errors for failed or dropped registrations and keep late
  completion isolated from replacement environments.

## Testing

- Add coverage for readiness gating, registration failure and replacement,
  eager Noise connections, and reconnection through a fresh rendezvous bundle.

GitOrigin-RevId: 83e23fa03a02e3b2bdf1a83fe26d7ac461f55cf3

codex-rs/exec-server/src/client.rs

@@ -453,8 +453,8 @@ impl LazyRemoteExecServerClient {
     fn can_reconnect(&self) -> bool {
         matches!(
             self.transport_params,
-            ExecServerTransportParams::WebSocketUrl { .. }
-                | ExecServerTransportParams::PendingWebSocketUrl(..)
+            ExecServerTransportParams::Deferred(_)
+                | ExecServerTransportParams::WebSocketUrl { .. }
                 | ExecServerTransportParams::NoiseRendezvous { .. }
         )
     }