[kotlin-client] Fix enum @JsonCreator to throw for unknown values instead of null, fixes #22662

[limoneren]

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Summary

• Fix enum decode() function with @JsonCreator to throw IllegalArgumentException for unknown enum values
• This regression was introduced in v7.18.0 via PR fix(kotlin): add JsonCreator/JsonValue to Jackson enums #22535
• The fix matches Java client and kotlin-spring generator behavior

Changes

• Modified enum_class.mustache to throw for unknown values when using Jackson
• Added unit tests for the new behavior
• Respects enumUnknownDefaultCase option

---

Summary by cubic

Fix Kotlin client enum deserialization with Jackson: decode(@JsonCreator) now throws IllegalArgumentException for unknown values (and for null when non-nullable). Restores pre-7.18.0 behavior and matches Java client and kotlin-spring; fixes #22662.

• Bug Fixes
  • Update enum_class.mustache to throw for unknown values; still honors enumUnknownDefaultCase for a default fallback and supports nullable enums.
  • Add tests to verify throwing behavior and the enumUnknownDefaultCase path.
  • Refresh Kotlin samples (spring-restclient and spring-webclient) to reflect the new decode logic.
  • Notes regression origin in v7.18.0 (PR fix(kotlin): add JsonCreator/JsonValue to Jackson enums #22535).

^{Written for commit c97a631. Summary will update on new commits.}

[cubic-dev-ai]

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