json : support enum values within allOf

[aldehir]

fix #15823

The allOf logic only supports object schemas with properties, this PR extends it to support enum as well.

Strictly speaking, combining allOf with enum should only consider the intersect of all sub-schemas. This PR violates that by aggregating them to minimize complexity. I can add the intersection if desired, I just didn't think it was worth the hassle.

[deiteris]

I'm not sure the current implementation considers the intersection even for properties (with additionalProperties: false there must be an intersection as well, which can be a pita). From the correctness standpoint, intersection is a must here, otherwise the resulting grammar will not be equivalent to the rule you define, but ofc it depends on expected usage. For example the following schema:

{
  "allOf": [
      {"$ref": "#/definitions/foo"},
      {"$ref": "#/definitions/bar"}
  ],
  "definitions": {
      "foo": {
          "type": "string",
          "enum": ["a", "b"]
      },
      "bar": {
          "type": "string",
          "enum": ["b", "c"]
      }
  }
}

In your case will produce an aggregate of enums and your resulting rule will be able to match "a", "b", "b", and "c". But if you validate the same data against the original schema using a compliant JSON Schema validator, only "b" will pass.

[directrix1]

This fixed my issue with using Zed editor which was defining tool calls with this kind of schema.

[aldehir]

For the sake of correctness, I added the intersection logic. That said, I highly doubt clients are going to add multiple enum schemas.