Add stdio exec-server listener (#20663)

## Why

This stack adds configured exec-server environments, including
environments reached over stdio. Before client-side stdio transports or
config can use that path, the exec-server binary itself needs a
first-class stdio listen mode so it can speak the same JSON-RPC protocol
over stdin/stdout that it already speaks over websockets.

**Stack position:** this is PR 1 of 5. It is the server-side transport
foundation for the stack.

## What Changed

- Accept `stdio` and `stdio://` for `codex exec-server --listen`.
- Promote the existing stdio `JsonRpcConnection` helper from test-only
code into normal exec-server transport code.
- Add parse coverage for stdio listen URLs while preserving the existing
websocket default.

## Stack

- **1. This PR:** https://github.com/openai/codex/pull/20663 - Add stdio
exec-server listener
- 2. 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.

---------

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

codex-rs/cli/src/main.rs

@@ -446,7 +446,7 @@ struct AppServerCommand {
 
 #[derive(Debug, Parser)]
 struct ExecServerCommand {
-    /// Transport endpoint URL. Supported values: `ws://IP:PORT` (default).
+    /// Transport endpoint URL. Supported values: `ws://IP:PORT` (default), `stdio`, `stdio://`.
     #[arg(
         long = "listen",
         value_name = "URL",