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/server/processor.rs

@@ -47,8 +47,13 @@ async fn run_connection(
     runtime_paths: ExecServerRuntimePaths,
 ) {
     let router = Arc::new(build_router());
-    let (json_outgoing_tx, mut incoming_rx, mut disconnected_rx, connection_tasks) =
-        connection.into_parts();
+    let JsonRpcConnection {
+        outgoing_tx: json_outgoing_tx,
+        mut incoming_rx,
+        mut disconnected_rx,
+        task_handles: connection_tasks,
+        transport: _transport,
+    } = connection;
     let (outgoing_tx, mut outgoing_rx) =
         mpsc::channel::<RpcServerOutboundMessage>(CHANNEL_CAPACITY);
     let notifications = RpcNotificationSender::new(outgoing_tx.clone());