[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/session/mcp.rs

@@ -205,6 +205,13 @@ impl Session {
             turn_context.sub_id.clone(),
             self.get_tx_event(),
             turn_context.sandbox_policy.get().clone(),
+            McpRuntimeEnvironment::new(
+                turn_context
+                    .environment
+                    .clone()
+                    .unwrap_or_else(|| Arc::new(Environment::default())),
+                turn_context.cwd.to_path_buf(),
+            ),
             config.codex_home.to_path_buf(),
             codex_apps_tools_cache_key(auth.as_ref()),
             tool_plugin_provenance,

codex-rs/core/src/session/mod.rs

@@ -55,6 +55,7 @@ use codex_login::CodexAuth;
 use codex_login::auth_env_telemetry::collect_auth_env_telemetry;
 use codex_login::default_client::originator;
 use codex_mcp::McpConnectionManager;
+use codex_mcp::McpRuntimeEnvironment;
 use codex_mcp::ToolInfo;
 use codex_mcp::codex_apps_tools_cache_key;
 #[cfg(test)]

codex-rs/core/src/session/session.rs

@@ -676,7 +676,7 @@ impl Session {
             code_mode_service: crate::tools::code_mode::CodeModeService::new(
                 config.js_repl_node_path.clone(),
             ),
-            environment,
+            environment: environment.clone(),
         };
         services
             .model_client
@@ -770,6 +770,12 @@ impl Session {
             INITIAL_SUBMIT_ID.to_owned(),
             tx_event.clone(),
             session_configuration.sandbox_policy.get().clone(),
+            McpRuntimeEnvironment::new(
+                environment
+                    .clone()
+                    .unwrap_or_else(|| Arc::new(Environment::default())),
+                session_configuration.cwd.to_path_buf(),
+            ),
             config.codex_home.to_path_buf(),
             codex_apps_tools_cache_key(auth),
             tool_plugin_provenance,