import @Valid in Spring delegates

[martin-mfg]

Fix #15213 by importing @Valid in Spring Delegates.

The import is added even if the @Valid annotation is not actually used in the generated file. Since the generated files already contain several unused imports, I think that's ok for now.

Should I add a new config under /bin/configs which uses spring generator with delegatePattern=true and the OpenAPI spec from #15213, to avoid regressions?

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[borsch]

@MelleD seems like issue related to your changes. Can we fix it somehow without adding extract import?