[metaschema] Invalid attribute sibling property use is not detected #1737

Invalid attribute sibling property use is not detected in the metaschema.

metaschema/attribute.schema.json allows the sibling property to be used in any attribute definition, not just enum attributes.

We can fix this in the attribute dictionary. In metaschema/dictionary-attribute.schema.json, add the following:

{
  "additionalProperties": false,
  // ...
  // Add "dependentRequired" as a top-level property
  "dependentRequired": {
    "sibling": [
      "enum"
    ]
  }
}

This covers the most important actual usage case while maintaining support for existing cases where the sibling property exists in class (event) and object definitions.

This solution does not prevent adding the sibling attribute property in other situations where it does not apply, such as profile, class, and object attributes that are not enum attributes.