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.rs

@@ -895,6 +895,8 @@ mod tests {
     use super::ExecServerClientConnectOptions;
     use crate::ProcessId;
     #[cfg(not(windows))]
+    use crate::client_api::DEFAULT_REMOTE_EXEC_SERVER_INITIALIZE_TIMEOUT;
+    #[cfg(not(windows))]
     use crate::client_api::ExecServerTransportParams;
     use crate::client_api::StdioExecServerCommand;
     use crate::client_api::StdioExecServerConnectArgs;
@@ -962,15 +964,18 @@ mod tests {
     #[tokio::test]
     async fn connect_for_transport_initializes_stdio_command() {
         let client = ExecServerClient::connect_for_transport(
-            ExecServerTransportParams::StdioCommand(StdioExecServerCommand {
-                program: "sh".to_string(),
-                args: vec![
-                    "-c".to_string(),
-                    "read _line; printf '%s\\n' '{\"id\":1,\"result\":{\"sessionId\":\"stdio-test\"}}'; read _line; sleep 60".to_string(),
-                ],
-                env: HashMap::new(),
-                cwd: None,
-            }),
+            ExecServerTransportParams::StdioCommand {
+                command: StdioExecServerCommand {
+                    program: "sh".to_string(),
+                    args: vec![
+                        "-c".to_string(),
+                        "read _line; printf '%s\\n' '{\"id\":1,\"result\":{\"sessionId\":\"stdio-test\"}}'; read _line; sleep 60".to_string(),
+                    ],
+                    env: HashMap::new(),
+                    cwd: None,
+                },
+                initialize_timeout: DEFAULT_REMOTE_EXEC_SERVER_INITIALIZE_TIMEOUT,
+            },
         )
         .await
         .expect("stdio transport should connect");