[4/6] Abstract MCP stdio server launching (#18087)

## Summary
- Move local MCP stdio process startup behind a launcher trait.
- Preserve existing local stdio behavior while making transport creation
explicit.

## Stack
```text
o  #18027 [6/6] Fail exec client operations after disconnect
│
o  #18212 [5/6] Wire executor-backed MCP stdio
│
@  #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

@@ -5,6 +5,7 @@ mod oauth;
 mod perform_oauth_login;
 mod program_resolver;
 mod rmcp_client;
+mod stdio_server_launcher;
 mod utils;
 
 pub use auth_status::StreamableHttpOAuthDiscovery;
@@ -29,3 +30,5 @@ 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::LocalStdioServerLauncher;
+pub use stdio_server_launcher::StdioServerLauncher;