[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/codex-mcp/src/mcp_connection_manager.rs

@@ -50,8 +50,10 @@ use codex_protocol::protocol::McpStartupStatus;
 use codex_protocol::protocol::McpStartupUpdateEvent;
 use codex_protocol::protocol::SandboxPolicy;
 use codex_rmcp_client::ElicitationResponse;
+use codex_rmcp_client::LocalStdioServerLauncher;
 use codex_rmcp_client::RmcpClient;
 use codex_rmcp_client::SendElicitation;
+use codex_rmcp_client::StdioServerLauncher;
 use futures::future::BoxFuture;
 use futures::future::FutureExt;
 use futures::future::Shared;
@@ -1500,7 +1502,8 @@ async fn make_rmcp_client(
                     .map(|(key, value)| (key.into(), value.into()))
                     .collect::<HashMap<_, _>>()
             });
-            RmcpClient::new_stdio_client(command_os, args_os, env_os, &env_vars, cwd)
+            let launcher = Arc::new(LocalStdioServerLauncher) as Arc<dyn StdioServerLauncher>;
+            RmcpClient::new_stdio_client(command_os, args_os, env_os, &env_vars, cwd, launcher)
                 .await
                 .map_err(|err| StartupOutcomeError::from(anyhow!(err)))
         }