Add stdio exec-server client transport (#20664)

## Why

Configured environments need to connect to exec-server instances that
are not necessarily already listening on a websocket URL. A
command-backed stdio transport lets Codex start an exec-server process,
speak JSON-RPC over its stdio streams, and clean up that child process
with the client lifetime.

**Stack position:** this is PR 2 of 5. It builds on the server-side
stdio listener from PR 1 and provides the client transport used by later
environment/config PRs.

## What Changed

- Add `ExecServerTransport` variants for websocket URLs and stdio shell
commands.
- Add stdio command connection support for `ExecServerClient`.
- Move websocket/stdio transport setup into `client_transport.rs` so
`client.rs` stays focused on shared JSON-RPC client, session, HTTP, and
notification behavior.
- Tie stdio child process cleanup to the JSON-RPC connection lifetime
with a RAII lifetime guard.
- Keep existing websocket environment behavior by adapting URL-backed
remotes to `ExecServerTransport::WebSocketUrl`.

## Stack

- 1. https://github.com/openai/codex/pull/20663 - Add stdio exec-server
listener
- **2. This PR:** https://github.com/openai/codex/pull/20664 - Add stdio
exec-server client transport
- 3. https://github.com/openai/codex/pull/20665 - Make environment
providers own default selection
- 4. https://github.com/openai/codex/pull/20666 - Add CODEX_HOME
environments TOML provider
- 5. https://github.com/openai/codex/pull/20667 - Load configured
environments from CODEX_HOME

Split from original draft: https://github.com/openai/codex/pull/20508

## Validation

Not run locally; this was split out of the original draft stack and then
refactored to separate transport setup from the base client.

---------

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

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

@@ -7,6 +7,7 @@ use crate::ExecutorFileSystem;
 use crate::HttpClient;
 use crate::client::LazyRemoteExecServerClient;
 use crate::client::http_client::ReqwestHttpClient;
+use crate::client_api::ExecServerTransportParams;
 use crate::environment_provider::DefaultEnvironmentProvider;
 use crate::environment_provider::EnvironmentProvider;
 use crate::environment_provider::normalize_exec_server_url;
@@ -274,7 +275,9 @@ impl Environment {
         exec_server_url: String,
         local_runtime_paths: Option<ExecServerRuntimePaths>,
     ) -> Self {
-        let client = LazyRemoteExecServerClient::new(exec_server_url.clone());
+        let client = LazyRemoteExecServerClient::new(ExecServerTransportParams::WebSocketUrl(
+            exec_server_url.clone(),
+        ));
         let exec_backend: Arc<dyn ExecBackend> = Arc::new(RemoteProcess::new(client.clone()));
         let filesystem: Arc<dyn ExecutorFileSystem> =
             Arc::new(RemoteFileSystem::new(client.clone()));