[3/3] app-server: configure environment connection timeout (#29025)

## Why

Remote environments registered through `environment/add` currently use
the fixed 10-second WebSocket connection timeout. Slow-starting
executors need a caller-selected connection window, but this should not
add retry policy or couple exec-server behavior to Core’s
`deferred_executor` feature.

Make the timeout an optional part of the existing experimental request.
Existing clients continue using the current default, while callers that
know an executor may take longer can request a larger window explicitly.

Depends on #28683.

## What changed

- Add optional `connectTimeoutMs` to `EnvironmentAddParams` and document
it in the app-server README.
- Pass the optional timeout through `EnvironmentRequestProcessor` into
one `EnvironmentManager::upsert_environment()` path; the manager applies
the existing default when it is omitted.
- Preserve the existing single-attempt lifecycle. The configured value
controls WebSocket connection and handshake time for both initial
connection and later reconnects; initialization retains its separate
timeout.
- Add an app-server integration test that sends the real JSON-RPC
request and verifies a stalled handshake observes the requested timeout.

## Test plan

- `just test -p codex-app-server-protocol`
- `just test -p codex-exec-server`
- `just test -p codex-app-server
environment_add_applies_connect_timeout`

## Rollout

This is additive and does not enable `deferred_executor`. Callers should
send a non-default timeout only after a compatible app-server is
deployed; omitted or `null` values retain the existing 10-second
default.

codex-rs/core/src/environment_selection.rs

@@ -700,6 +700,7 @@ url = "ws://127.0.0.1:8765"
             .upsert_environment(
                 REMOTE_ENVIRONMENT_ID.to_string(),
                 format!("ws://{}", listener.local_addr().expect("listener address")),
+                /*connect_timeout*/ None,
             )
             .expect("replacement environment");
         environments.update_selections(std::slice::from_ref(&selection));
@@ -753,6 +754,7 @@ url = "ws://127.0.0.1:8765"
                         .local_addr()
                         .expect("second listener address")
                 ),
+                /*connect_timeout*/ None,
             )
             .expect("replace environment");
 
@@ -802,6 +804,7 @@ url = "ws://127.0.0.1:8765"
             .upsert_environment(
                 REMOTE_ENVIRONMENT_ID.to_string(),
                 "ws://127.0.0.1:9876".to_string(),
+                /*connect_timeout*/ None,
             )
             .expect("replacement environment");
         let environments = ThreadEnvironments::new(