[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/config.schema.json

@@ -876,6 +876,28 @@
       ],
       "type": "string"
     },
+    "McpServerEnvVar": {
+      "anyOf": [
+        {
+          "type": "string"
+        },
+        {
+          "additionalProperties": false,
+          "properties": {
+            "name": {
+              "type": "string"
+            },
+            "source": {
+              "type": "string"
+            }
+          },
+          "required": [
+            "name"
+          ],
+          "type": "object"
+        }
+      ]
+    },
     "McpServerToolConfig": {
       "additionalProperties": false,
       "description": "Per-tool approval settings for a single MCP server tool.",
@@ -1570,7 +1592,7 @@
         "env_vars": {
           "default": null,
           "items": {
-            "type": "string"
+            "$ref": "#/definitions/McpServerEnvVar"
           },
           "type": "array"
         },