[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/app-server/src/request_processors/environment_processor.rs

@@ -1,4 +1,5 @@
 use super::*;
+use std::time::Duration;
 
 #[derive(Clone)]
 pub(crate) struct EnvironmentRequestProcessor {
@@ -17,7 +18,11 @@ impl EnvironmentRequestProcessor {
         params: EnvironmentAddParams,
     ) -> Result<Option<ClientResponsePayload>, JSONRPCErrorError> {
         self.environment_manager
-            .upsert_environment(params.environment_id, params.exec_server_url)
+            .upsert_environment(
+                params.environment_id,
+                params.exec_server_url,
+                params.connect_timeout_ms.map(Duration::from_millis),
+            )
             .map_err(|err| invalid_request(err.to_string()))?;
         Ok(Some(EnvironmentAddResponse {}.into()))
     }