Disallow unknown keywords in meta-schema

[gregsdennis]

As a follow-up to #1610, there has been some discussion in Slack around a proposal I made for a way to allow for extensions to the meta-schema while also disallowing unknown keywords with unevaluatedProperties: false.

After iterating on the idea, we landed on this structure for the meta-schema:

{
  "$schema": "https://json-schema.org/1/2025",
  "$id": "https://json-schema.org/1/2025",

  "type": ["object", "boolean"],
  "properties": {
    "$schema": { "type": "string", "format": "iri" },
    "additionalProperties": { "$ref": "#" } // showing for self-reference
    // ...
  },
  "unevaluatedProperties": false,

  "$dynamicRef": "#extension",

  "$defs": {
    "extension": {
      "$dynamicAnchor": "extension"      
    }
  }
}

Then an extension meta-schema would need to look like this:

{
  "$schema": "https://json-schema.org/1/2025",
  "$id": "https://example.com/custom",
  
  "$ref": "https://json-schema.org/1/2025",

  "$defs": {
    "extension": {
      "$dynamicAnchor": "extension",
      "properties": {
        "foo": true
      }
    }
  }
}

Tracing this through:

1. Start validation with the custom meta-schema, https://example.com/custom. This registers the $dynamicAnchor.
2. The custom meta-schema references the base meta-schema (ours).
3. The base meta-schema has a $dynamicRef: #extension that resolves back to the definition in the custom meta-schema, where any additional properties are defined.
4. Because these extra properties are now defined via the $dynamicRef applicator, unevaluatedProperties can see them and will know to ignore them.
5. Lastly, applicators can once again be defined using just $ref: # (like draft 7 and earlier) because the reference back to the custom meta-schema is handled by the base meta-schema root.

It does feel a bit awkward to put your extension information in a $defs entry, but then you're also making a custom meta-schema, which isn't something I expect many people will do.