🐛 Handle omitzero in OneOf constraint validation

[HadrienPatte]

The OneOf/AtMostOneOf/AtLeastOneOf constraint validation rejects fields that lack the omitempty json tag, since such fields would be marked as required, conflicting with being part of a OneOf group.

However, omitzero (supported since Go 1.24 via encoding/json) serves the same serialization purpose: the field is omitted when it holds its zero value. The current check does not recognize omitzero, causing a false positive error for valid field declarations.

Example false positive error:

field endpointSelector is part of OneOf constraint and must have omitempty tag

[k8s-ci-robot]

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[JoelSpeed]

Change seems reasonable to me, but can you please add some tests to demonstrate the new behaviour and prevent future regressions

[HadrienPatte]

Updated to add tests