Increase exec-server environment transport timeouts (#21825)

## Why

The environment-backed exec-server transport currently hardcodes 5
second connect and initialize timeouts in `client_transport.rs`. That is
short for SSH-backed stdio environments and remote websocket
environments, and there is currently no way to raise those values from
`CODEX_HOME/environments.toml`.

This stacked follow-up raises the default environment transport timeouts
and lets each configured environment override them in
`environments.toml`.

## What Changed

- raise the default environment transport connect and initialize
timeouts from 5s to 10s
- store concrete timeout values on `ExecServerTransportParams` instead
of hardcoding them in `connect_for_transport(...)`
- add `connect_timeout_sec` and `initialize_timeout_sec` to
`[[environments]]` entries in `environments.toml`
- apply parse-time defaults so runtime transport code receives fully
resolved timeout values
- reject `connect_timeout_sec` on stdio environments because it only
applies to websocket transports
- extend parser tests to cover the new fields and defaults

## Stack

- base: https://github.com/openai/codex/pull/21794
- this PR: configurable environment transport timeouts

## Validation

- `cd
/Users/starr/code/codex-worktrees/exec-env-timeouts-config-20260508/codex-rs
&& just fmt`
- not run: tests

---------

Co-authored-by: Codex <noreply@openai.com>

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

@@ -373,7 +373,7 @@ impl Environment {
         local_runtime_paths: Option<ExecServerRuntimePaths>,
     ) -> Self {
         Self::remote_with_transport(
-            ExecServerTransportParams::WebSocketUrl(exec_server_url),
+            ExecServerTransportParams::websocket_url(exec_server_url),
             local_runtime_paths,
         )
     }
@@ -383,10 +383,11 @@ impl Environment {
         local_runtime_paths: Option<ExecServerRuntimePaths>,
     ) -> Self {
         let exec_server_url = match &remote_transport {
-            ExecServerTransportParams::WebSocketUrl(exec_server_url) => {
-                Some(exec_server_url.clone())
-            }
-            ExecServerTransportParams::StdioCommand(_) => None,
+            ExecServerTransportParams::WebSocketUrl {
+                websocket_url: exec_server_url,
+                ..
+            } => Some(exec_server_url.clone()),
+            ExecServerTransportParams::StdioCommand { .. } => None,
         };
         let client = LazyRemoteExecServerClient::new(remote_transport.clone());
         let exec_backend: Arc<dyn ExecBackend> = Arc::new(RemoteProcess::new(client.clone()));