[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/core/src/connectors.rs

@@ -13,6 +13,7 @@ pub use codex_app_server_protocol::AppInfo;
 pub use codex_app_server_protocol::AppMetadata;
 use codex_connectors::AllConnectorsCacheKey;
 use codex_connectors::DirectoryListResponse;
+use codex_exec_server::Environment;
 use codex_login::token_data::TokenData;
 use codex_protocol::protocol::SandboxPolicy;
 use codex_tools::DiscoverableTool;
@@ -37,6 +38,7 @@ use codex_login::default_client::create_client;
 use codex_login::default_client::originator;
 use codex_mcp::CODEX_APPS_MCP_SERVER_NAME;
 use codex_mcp::McpConnectionManager;
+use codex_mcp::McpRuntimeEnvironment;
 use codex_mcp::ToolInfo;
 use codex_mcp::ToolPluginProvenance;
 use codex_mcp::codex_apps_tools_cache_key;
@@ -241,6 +243,7 @@ pub async fn list_accessible_connectors_from_mcp_tools_with_options_and_status(
         INITIAL_SUBMIT_ID.to_owned(),
         tx_event,
         SandboxPolicy::new_read_only_policy(),
+        McpRuntimeEnvironment::new(Arc::new(Environment::default()), config.cwd.to_path_buf()),
         config.codex_home.to_path_buf(),
         codex_apps_tools_cache_key(auth.as_ref()),
         ToolPluginProvenance::default(),