[codex] disable Nagle on Rendezvous WebSockets (#30269)

## Summary

Disable Nagle unconditionally for both exec-server Rendezvous WebSocket
connections.

- pass `disable_nagle=true` at the executor and harness connection call
sites
- keep the existing signed URL, protocol, and connection flow unchanged
- add no feature flag, rollout schema, path variant, or
experiment-specific telemetry

The companion internal PR enables `TCP_NODELAY` on accepted Rendezvous
sockets: https://github.com/openai/openai/pull/1082463

## Why

Rendezvous carries small, latency-sensitive relay and JSON-RPC frames.
Three staging runs of 30 steady-state `process/read` calls per
configuration measured p50 improving from 139.1 ms to 81.5 ms and p95
from 162.0 ms to 95.8 ms with Nagle disabled.

The expected packet overhead is small at the current connection scale.
We will use existing latency, error, packet, and CPU monitoring and
revert normally if production regresses.

## Rollout and rollback

The client and accepted-socket changes can deploy independently. New
connections receive the setting as each side deploys. Rollback is a
normal code revert; there is no persisted assignment or gate state to
unwind.

## Validation

- `just test -p codex-exec-server --lib`: 164 passed
- `just fix -p codex-exec-server`: passed
- `just fmt`: passed
- independent final review found no actionable issue

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

@@ -274,7 +274,9 @@ impl ExecServerClient {
             connect_async_with_config(
                 request,
                 Some(noise_relay_websocket_config()),
-                /*disable_nagle*/ false,
+                // Rendezvous sends small, latency-sensitive frames, so avoid Nagle's coalescing delay.
+                /*disable_nagle*/
+                true,
             ),
         )
         .await

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

@@ -561,7 +561,9 @@ async fn connect_rendezvous(
         connect_async_with_config(
             request,
             Some(noise_relay_websocket_config()),
-            /*disable_nagle*/ false,
+            // Rendezvous sends small, latency-sensitive frames, so avoid Nagle's coalescing delay.
+            /*disable_nagle*/
+            true,
         )
         .await
         .map(|(websocket, _)| websocket)