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/client_api.rs

@@ -1,3 +1,5 @@
+use std::collections::HashMap;
+use std::path::PathBuf;
 use std::time::Duration;
 
 use futures::future::BoxFuture;
@@ -25,6 +27,32 @@ pub struct RemoteExecServerConnectArgs {
     pub resume_session_id: Option<String>,
 }
 
+/// Stdio connection arguments for a command-backed exec-server.
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub(crate) struct StdioExecServerConnectArgs {
+    pub command: StdioExecServerCommand,
+    pub client_name: String,
+    pub initialize_timeout: Duration,
+    pub resume_session_id: Option<String>,
+}
+
+/// Structured process command used to start an exec-server over stdio.
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub(crate) struct StdioExecServerCommand {
+    pub program: String,
+    pub args: Vec<String>,
+    pub env: HashMap<String, String>,
+    pub cwd: Option<PathBuf>,
+}
+
+/// Parameters used to connect to a remote exec-server environment.
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub(crate) enum ExecServerTransportParams {
+    WebSocketUrl(String),
+    #[allow(dead_code)]
+    StdioCommand(StdioExecServerCommand),
+}
+
 /// Sends HTTP requests through a runtime-selected transport.
 ///
 /// This is the HTTP capability counterpart to [`crate::ExecBackend`]. Callers