The validation code generator was failing to propagate the
StabilityLevel from the parent context when evaluating composite tags
like `+k8s:item`, `+k8s:each`, and `+k8s:subfield`. As a result, inner
validations like `+k8s:zeroOrOneOfMember` combined with `+k8s:alpha`
were not emitting `.MarkAlpha()` in the generated code.
This fix ensures the `StabilityLevel` is copied to the `subContext`
during evaluation, and adds a test case for list items using
`+k8s:alpha=+k8s:item(...)=+k8s:zeroOrOneOfMember`.
Generated code is updated accordingly.