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]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: bryantbiggs
Once this PR has been reviewed and has the lgtm label, please assign apelisse for approval. For more information see the Code Review Process.

The full list of commands accepted by this bot can be found here.

Details

Needs approval from an approver in each of these files:

• pkg/util/proto/OWNERS

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[Jefftree]

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

[k8s-triage-robot]

The Kubernetes project currently lacks enough contributors to adequately respond to all PRs.

This bot triages PRs according to the following rules:

• After 90d of inactivity, lifecycle/stale is applied
• After 30d of inactivity since lifecycle/stale was applied, lifecycle/rotten is applied
• After 30d of inactivity since lifecycle/rotten was applied, the PR is closed

You can:

• Mark this PR as fresh with /remove-lifecycle stale
• Close this PR with /close
• Offer to help out with Issue Triage

Please send feedback to sig-contributor-experience at kubernetes/community.

/lifecycle stale