OAS 3.1/3.2 discriminator is annotation-only

[p1c2u]

The OAS 3.1 validator was inheriting custom oneOf/anyOf/allOf handlers that replace standard JSON Schema composition with discriminator-based $ref resolution (OAS 3.0 semantics). In OAS 3.1/3.2, discriminator is a pure annotation; composition validation is governed entirely by oneOf/anyOf/allOf. Removed those overrides so OAS 3.1/3.2 uses Draft 2020-12 semantics directly.

# Before: OAS31Validator with discriminator would silently attempt ref resolution
schema = {
    "$schema": "https://spec.openapis.org/oas/3.1/dialect/2024-11-10",
    "oneOf": [
        {"type": "object", "properties": {"petType": {"const": "cat"}, "meow": {"type": "string"}}, "required": ["petType", "meow"]},
    ],
    "discriminator": {"propertyName": "petType"},
}
v = OAS31Validator(schema)
list(v.iter_errors({"petType": "cat", "meow": "purr"}))
# Was: [ValidationError: "... reference '#/components/schemas/cat' could not be resolved"]
# Now: []  — discriminator ignored, oneOf validates normally