What type of PR is this?

/kind feature

What this PR does / why we need it:

This PR enhances the validation logic for union types in the Kubernetes API machinery. By adding .WithOrigin("union") to the validation error returns, it provides more specific and actionable error messages. When a union validation fails (e.g., because multiple fields are set where only one is allowed), the error message will now clearly indicate that the "union" validation was the source of the failure. This makes it easier for developers to debug issues related to union type validation.

The changes are applied consistently across various union validation scenarios, including discriminated and undiscriminated unions, as shown by the updates to the validation logic and the corresponding test files.

Which issue(s) this PR is related to:

N/A

Special notes for your reviewer:

The core of this change is the addition of .WithOrigin("union") to the error construction in pkg/apis/resource/validation/validation.go and staging/src/k8s.io/apimachinery/pkg/api/validate/union.go. The majority of the file changes are in the test files to update the expected error matchers to include this new origin.

Does this PR introduce a user-facing change?

NONE

Additional documentation e.g., KEPs (Kubernetes Enhancement Proposals), usage docs, etc.:

NONE