[5/6] Wire executor-backed MCP stdio (#18212)

## Summary
- Add the executor-backed RMCP stdio transport.
- Wire MCP stdio placement through the executor environment config.
- Cover local and executor-backed stdio paths with the existing MCP test
helpers.

## Stack
```text
o  #18027 [6/6] Fail exec client operations after disconnect
│
@  #18212 [5/6] Wire executor-backed MCP stdio
│
o  #18087 [4/6] Abstract MCP stdio server launching
│
o  #18020 [3/6] Add pushed exec process events
│
o  #18086 [2/6] Support piped stdin in exec process API
│
o  #18085 [1/6] Add MCP server environment config
│
o  main
```

---------

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

codex-rs/cli/tests/mcp_list.rs

@@ -55,7 +55,7 @@ async fn list_and_get_render_expected_output() -> Result<()> {
         .expect("docs server should exist after add");
     match &mut docs_entry.transport {
         McpServerTransportConfig::Stdio { env_vars, .. } => {
-            *env_vars = vec!["APP_TOKEN".to_string(), "WORKSPACE_ID".to_string()];
+            *env_vars = vec!["APP_TOKEN".into(), "WORKSPACE_ID".into()];
         }
         other => panic!("unexpected transport: {other:?}"),
     }