[kotlin-spring, java-spring] - gate @JsonSetter on openApiNullable for optional non-nullable fields

[Picazsoo]

This PR fixes #23976 by making @JsonSetter generation respect the openApiNullable decision agreed in the issue discussion.

Summary of changes

• For optional, non-nullable properties (required: false, nullable: false):

  • openApiNullable=false now generates @JsonSetter(nulls = Nulls.SKIP), so explicit JSON null is ignored and existing/default values are protected.
  • openApiNullable=true keeps/generates @JsonSetter(nulls = Nulls.FAIL), so explicit JSON null is rejected for stricter PATCH-style semantics.

• The Kotlin Spring generator was updated to stop unconditionally emitting Nulls.FAIL for optional non-nullable fields.

• Java Spring was aligned for the openApiNullable=false case by adding @JsonSetter(nulls = Nulls.SKIP) to generated setters, including the required Jackson imports. This is especially important for optional non-nullable attributes with defaults where it was possible to overwrite the expected default.

• Tests and sample outputs were updated to cover:

  • Kotlin Spring behavior with and without openApiNullable
  • default-value protection via Nulls.SKIP
  • Jackson 3 import behavior
  • regenerated Kotlin Spring and Java Spring samples

• Kotlin Spring documentation for openApiNullable was expanded to explain the new strict-vs-lenient null handling behavior.

  Co-authored-by: Copilot 223556219+Copilot@users.noreply.github.com

PR checklist

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It would be really good to get some java-spring feedback for the Nulls.SKIP for optional non-nullable.

---

Summary by cubic

Gated @JsonSetter generation for optional, non-nullable fields in Kotlin Spring and Java Spring based on openApiNullable. Restores lenient handling when openApiNullable=false and keeps strict null rejection when openApiNullable=true.

• Bug Fixes
  • Optional non-nullable fields now respect openApiNullable: openApiNullable=false → @JsonSetter(nulls = Nulls.SKIP); openApiNullable=true → @JsonSetter(nulls = Nulls.FAIL).
  • Java Spring now emits @JsonSetter(nulls = Nulls.SKIP) for the same case. Added JsonSetter/Nulls imports from com.fasterxml.jackson.annotation and updated templates, tests, and samples.
  • Updated Kotlin Spring docs for openApiNullable to clarify strict-null behavior and requirements.

^{Written for commit 0f44be5. Summary will update on new commits.}

[cubic-dev-ai]

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[Picazsoo]

Tagging you @mfunda-cen59049 for review as agreed upon.

[mfunda-cen59049]

Thanks for providing a fix for the issue so quickly. I went over the code changes and this will help a lot with our current use-case.