[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/mcp/mod.rs

@@ -35,6 +35,7 @@ use codex_protocol::protocol::SandboxPolicy;
 use serde_json::Value;
 
 use crate::mcp_connection_manager::McpConnectionManager;
+use crate::mcp_connection_manager::McpRuntimeEnvironment;
 use crate::mcp_connection_manager::codex_apps_tools_cache_key;
 pub type McpManager = McpConnectionManager;
 
@@ -321,14 +322,23 @@ pub async fn collect_mcp_snapshot(
     config: &McpConfig,
     auth: Option<&CodexAuth>,
     submit_id: String,
+    runtime_environment: McpRuntimeEnvironment,
 ) -> McpListToolsResponseEvent {
-    collect_mcp_snapshot_with_detail(config, auth, submit_id, McpSnapshotDetail::Full).await
+    collect_mcp_snapshot_with_detail(
+        config,
+        auth,
+        submit_id,
+        runtime_environment,
+        McpSnapshotDetail::Full,
+    )
+    .await
 }
 
 pub async fn collect_mcp_snapshot_with_detail(
     config: &McpConfig,
     auth: Option<&CodexAuth>,
     submit_id: String,
+    runtime_environment: McpRuntimeEnvironment,
     detail: McpSnapshotDetail,
 ) -> McpListToolsResponseEvent {
     let mcp_servers = effective_mcp_servers(config, auth);
@@ -356,6 +366,7 @@ pub async fn collect_mcp_snapshot_with_detail(
         submit_id,
         tx_event,
         SandboxPolicy::new_read_only_policy(),
+        runtime_environment,
         config.codex_home.clone(),
         codex_apps_tools_cache_key(auth),
         tool_plugin_provenance,
@@ -386,15 +397,23 @@ pub async fn collect_mcp_server_status_snapshot(
     config: &McpConfig,
     auth: Option<&CodexAuth>,
     submit_id: String,
+    runtime_environment: McpRuntimeEnvironment,
 ) -> McpServerStatusSnapshot {
-    collect_mcp_server_status_snapshot_with_detail(config, auth, submit_id, McpSnapshotDetail::Full)
-        .await
+    collect_mcp_server_status_snapshot_with_detail(
+        config,
+        auth,
+        submit_id,
+        runtime_environment,
+        McpSnapshotDetail::Full,
+    )
+    .await
 }
 
 pub async fn collect_mcp_server_status_snapshot_with_detail(
     config: &McpConfig,
     auth: Option<&CodexAuth>,
     submit_id: String,
+    runtime_environment: McpRuntimeEnvironment,
     detail: McpSnapshotDetail,
 ) -> McpServerStatusSnapshot {
     let mcp_servers = effective_mcp_servers(config, auth);
@@ -422,6 +441,7 @@ pub async fn collect_mcp_server_status_snapshot_with_detail(
         submit_id,
         tx_event,
         SandboxPolicy::new_read_only_policy(),
+        runtime_environment,
         config.codex_home.clone(),
         codex_apps_tools_cache_key(auth),
         tool_plugin_provenance,