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/client_api.rs

@@ -9,6 +9,9 @@ use crate::HttpRequestParams;
 use crate::HttpRequestResponse;
 use crate::HttpResponseBodyStream;
 
+pub(crate) const DEFAULT_REMOTE_EXEC_SERVER_CONNECT_TIMEOUT: Duration = Duration::from_secs(10);
+pub(crate) const DEFAULT_REMOTE_EXEC_SERVER_INITIALIZE_TIMEOUT: Duration = Duration::from_secs(10);
+
 /// Connection options for any exec-server client transport.
 #[derive(Debug, Clone, PartialEq, Eq)]
 pub struct ExecServerClientConnectOptions {
@@ -48,9 +51,26 @@ pub(crate) struct StdioExecServerCommand {
 /// Parameters used to connect to a remote exec-server environment.
 #[derive(Debug, Clone, PartialEq, Eq)]
 pub(crate) enum ExecServerTransportParams {
-    WebSocketUrl(String),
+    WebSocketUrl {
+        websocket_url: String,
+        connect_timeout: Duration,
+        initialize_timeout: Duration,
+    },
     #[allow(dead_code)]
-    StdioCommand(StdioExecServerCommand),
+    StdioCommand {
+        command: StdioExecServerCommand,
+        initialize_timeout: Duration,
+    },
+}
+
+impl ExecServerTransportParams {
+    pub(crate) fn websocket_url(websocket_url: String) -> Self {
+        Self::WebSocketUrl {
+            websocket_url,
+            connect_timeout: DEFAULT_REMOTE_EXEC_SERVER_CONNECT_TIMEOUT,
+            initialize_timeout: DEFAULT_REMOTE_EXEC_SERVER_INITIALIZE_TIMEOUT,
+        }
+    }
 }
 
 /// Sends HTTP requests through a runtime-selected transport.