[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/config/src/mcp_edit.rs

@@ -14,6 +14,7 @@ use toml_edit::value;
 use crate::AppToolApproval;
 use crate::CONFIG_TOML_FILE;
 use crate::McpServerConfig;
+use crate::McpServerEnvVar;
 use crate::McpServerTransportConfig;
 
 pub async fn load_global_mcp_servers(
@@ -142,7 +143,7 @@ fn serialize_mcp_server(config: &McpServerConfig) -> TomlItem {
                 entry["env"] = table_from_pairs(env.iter());
             }
             if !env_vars.is_empty() {
-                entry["env_vars"] = array_from_strings(env_vars);
+                entry["env_vars"] = array_from_env_vars(env_vars);
             }
             if let Some(cwd) = cwd {
                 entry["cwd"] = value(cwd.to_string_lossy().to_string());
@@ -247,6 +248,24 @@ fn array_from_strings(values: &[String]) -> TomlItem {
     TomlItem::Value(array.into())
 }
 
+fn array_from_env_vars(env_vars: &[McpServerEnvVar]) -> TomlItem {
+    let mut array = toml_edit::Array::new();
+    for env_var in env_vars {
+        match env_var {
+            McpServerEnvVar::Name(name) => array.push(name.clone()),
+            McpServerEnvVar::Config { name, source } => {
+                let mut table = toml_edit::InlineTable::new();
+                table.insert("name", name.clone().into());
+                if let Some(source) = source {
+                    table.insert("source", source.clone().into());
+                }
+                array.push(table);
+            }
+        }
+    }
+    TomlItem::Value(array.into())
+}
+
 fn table_from_pairs<'a, I>(pairs: I) -> TomlItem
 where
     I: IntoIterator<Item = (&'a String, &'a String)>,