[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/tests/process_group_cleanup.rs

@@ -4,10 +4,12 @@ use std::collections::HashMap;
 use std::ffi::OsString;
 use std::fs;
 use std::path::Path;
+use std::sync::Arc;
 use std::time::Duration;
 
 use anyhow::Context;
 use anyhow::Result;
+use codex_rmcp_client::LocalStdioServerLauncher;
 use codex_rmcp_client::RmcpClient;
 
 fn process_exists(pid: u32) -> bool {
@@ -78,6 +80,7 @@ async fn drop_kills_wrapper_process_group() -> Result<()> {
         )])),
         &[],
         /*cwd*/ None,
+        Arc::new(LocalStdioServerLauncher),
     )
     .await?;
 

codex-rs/rmcp-client/tests/resources.rs

@@ -1,9 +1,11 @@
 use std::ffi::OsString;
 use std::path::PathBuf;
+use std::sync::Arc;
 use std::time::Duration;
 
 use codex_rmcp_client::ElicitationAction;
 use codex_rmcp_client::ElicitationResponse;
+use codex_rmcp_client::LocalStdioServerLauncher;
 use codex_rmcp_client::RmcpClient;
 use codex_utils_cargo_bin::CargoBinError;
 use futures::FutureExt as _;
@@ -61,6 +63,7 @@ async fn rmcp_client_can_list_and_read_resources() -> anyhow::Result<()> {
         /*env*/ None,
         &[],
         /*cwd*/ None,
+        Arc::new(LocalStdioServerLauncher),
     )
     .await?;