Fix ParseSchemaV3 to handle allOf-wrapped $ref

[bryantbiggs]

Summary

• Fix ParseSchemaV3 in document_v3.go to handle the allOf-wrapped $ref pattern
• When a schema has no type but a single-element allOf containing a $ref, resolve it as a reference instead of falling through to Arbitrary

Context

In OpenAPI 3.0, sibling properties alongside $ref must be ignored per the spec. The compliant way to combine $ref with description/default is to wrap the $ref in a single-element allOf:

"metadata": {
  "allOf": [{"$ref": "#/components/schemas/ObjectMeta"}],
  "description": "Standard object's metadata.",
  "default": {}
}

kube-openapi's own builder (builder3/util.WrapRefs) produces this format, and Kubernetes >= 1.35 serves v3 specs using it. However, ParseSchemaV3 had zero handling for allOf — these schemas fell through to the default case and were returned as Arbitrary, losing the reference.

This matches the pattern already used in pkg/schemaconv/openapi.go:130-133.

Fixes #477
Unblocks #583

Test plan

• New test TestV3AllOfRef — verifies both direct $ref and allOf-wrapped $ref resolve as proto.Reference
• All 54 existing + new tests pass
• go vet clean
• CI verification

[k8s-ci-robot]

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[Jefftree]

Please fix the commit message. assuming it has to do with symbols being used.