[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/rmcp-client/src/lib.rs

@@ -1,5 +1,6 @@
 mod auth_status;
 mod elicitation_client_service;
+mod executor_process_transport;
 mod logging_client_handler;
 mod oauth;
 mod perform_oauth_login;
@@ -30,5 +31,6 @@ pub use rmcp_client::ListToolsWithConnectorIdResult;
 pub use rmcp_client::RmcpClient;
 pub use rmcp_client::SendElicitation;
 pub use rmcp_client::ToolWithConnectorId;
+pub use stdio_server_launcher::ExecutorStdioServerLauncher;
 pub use stdio_server_launcher::LocalStdioServerLauncher;
 pub use stdio_server_launcher::StdioServerLauncher;