Add MCP app feature flag (#19884)

## Summary
- Add the `enable_mcp_apps` feature flag to the `codex-features`
registry
- Keep it under development and disabled by default

## Testing
- Unit tests for `codex-features` passed
- Formatting passed

codex-rs/core/config.schema.json

@@ -400,6 +400,9 @@
             "enable_fanout": {
               "type": "boolean"
             },
+            "enable_mcp_apps": {
+              "type": "boolean"
+            },
             "enable_request_compression": {
               "type": "boolean"
             },
@@ -2607,6 +2610,9 @@
         "enable_fanout": {
           "type": "boolean"
         },
+        "enable_mcp_apps": {
+          "type": "boolean"
+        },
         "enable_request_compression": {
           "type": "boolean"
         },