[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/codex-mcp/src/lib.rs

@@ -38,6 +38,7 @@ pub use mcp_connection_manager::CodexAppsToolsCacheKey;
 pub use mcp_connection_manager::DEFAULT_STARTUP_TIMEOUT;
 pub use mcp_connection_manager::MCP_SANDBOX_STATE_META_CAPABILITY;
 pub use mcp_connection_manager::McpConnectionManager;
+pub use mcp_connection_manager::McpRuntimeEnvironment;
 pub use mcp_connection_manager::SandboxState;
 pub use mcp_connection_manager::ToolInfo;
 pub use mcp_connection_manager::codex_apps_tools_cache_key;