plugin-creator validator rejects supported .app.json required binding

Summary

The bundled plugin validator rejects the optional required field in an app binding, even though the Codex runtime accepts and enforces it.

Reproduction

Create a plugin with this app binding:

{
  "apps": {
    "sample-production": {
      "id": "asdk_app_example",
      "required": true
    }
  }
}

Run the bundled plugin-creator validator. It reports that the .app.json app field required is not accepted.

The relevant validator currently appears at:

codex-rs/skills/src/assets/samples/plugin-creator/scripts/validate_plugin.py

Expected behavior

The validator and plugin manifest specification should accept an optional boolean required field on app bindings, matching the runtime behavior.

Why this matters

Private production connectors can use a required app binding to fail closed instead of silently starting with the wrong or missing connector. The current mismatch creates a false validation failure for a runtime-supported safety configuration.

Actual behavior

The validator only permits id and category on the app entry and rejects required.