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/exec-server/src/connection.rs

@@ -8,13 +8,9 @@ use tokio::sync::watch;
 use tokio_tungstenite::WebSocketStream;
 use tokio_tungstenite::tungstenite::Message;
 
-#[cfg(test)]
 use tokio::io::AsyncBufReadExt;
-#[cfg(test)]
 use tokio::io::AsyncWriteExt;
-#[cfg(test)]
 use tokio::io::BufReader;
-#[cfg(test)]
 use tokio::io::BufWriter;
 
 pub(crate) const CHANNEL_CAPACITY: usize = 128;
@@ -34,7 +30,6 @@ pub(crate) struct JsonRpcConnection {
 }
 
 impl JsonRpcConnection {
-    #[cfg(test)]
     pub(crate) fn from_stdio<R, W>(reader: R, writer: W, connection_label: String) -> Self
     where
         R: AsyncRead + Unpin + Send + 'static,
@@ -298,7 +293,6 @@ async fn send_malformed_message(
         .await;
 }
 
-#[cfg(test)]
 async fn write_jsonrpc_line_message<W>(
     writer: &mut BufWriter<W>,
     message: &JSONRPCMessage,