[kotlin] Add @get:JsonValue to nested Jackson enums (#23886)

[seonwooj0810]

Fixes #23886

Problem

For the Kotlin client generator with serializationLibrary=jackson, nested (inline) enum classes generated into a model's data class are missing the @get:JsonValue annotation that top-level enum classes already carry (enum_class.mustache). Without it, Jackson serializes an Int-valued nested enum by its constant name instead of its numeric value, producing wrong JSON output (a string where a number is expected).

Change

• kotlin-client/data_class.mustache: add {{#jackson}}@get:JsonValue {{/jackson}} to the nested enum class declaration, mirroring enum_class.mustache, and import com.fasterxml.jackson.annotation.JsonValue (guarded by {{#hasEnums}}) so the generated code compiles.
• Added regression test KotlinClientCodegenModelTest#testJacksonNestedEnumsUseJsonValue with spec issue23886-kotlin-nested-numeric-enum.yaml.
• Regenerated the affected Kotlin Jackson client samples (./bin/generate-samples.sh for the kotlin-client jackson configs). All sample changes are limited to the added @get:JsonValue and the JsonValue import.

Test evidence

./mvnw -pl modules/openapi-generator test -Dtest='KotlinClientCodegenModelTest#testJacksonNestedEnumsUseJsonValue'
Tests run: 1, Failures: 0, Errors: 0, Skipped: 0
BUILD SUCCESS

Verification done: confirmed the gap on master (top-level enum_class.mustache emits @get:JsonValue while nested enums in data_class.mustache did not); confirmed no in-flight PR / claim on #23886; the diff across all 24 regenerated sample files contains only the new annotation/import lines.

PR checklist

• Read the contribution guidelines.
• Ran ./bin/generate-samples.sh for the affected kotlin-client jackson configs and committed all changed files.
• File the PR against the master branch.

@jimschubert @stefankoppier — Kotlin technical committee, this touches the kotlin-client enum template and the jvm-spring-webclient/jvm-spring-restclient samples.

---

Summary by cubic

Adds @get:JsonValue to nested enum value parameters in Kotlin Jackson client models so Jackson serializes by value (e.g., numbers) instead of enum name. Aligns nested enums with top‑level behavior and fixes #23886.

• Bug Fixes
  • kotlin-client/data_class.mustache: add @get:JsonValue to nested enum value; import com.fasterxml.jackson.annotation.JsonValue guarded by {{#hasEnums}}.
  • Tests: updated assertions to expect the annotation; added regression test KotlinClientCodegenModelTest#testJacksonNestedEnumsUseJsonValue using issue23886-kotlin-nested-numeric-enum.yaml.
  • Regenerated Kotlin Jackson client samples; changes limited to the new annotation and import.

^{Written for commit 675af99. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 27 files

_{Re-trigger cubic}

[seonwooj0810]

Rebased onto master; jackson kotlin samples regenerated and KotlinClientCodegenModelTest passes (51/51).

The only red check, Build Kotlin client (kotlin-bigdecimal-default-okhttp4), builds a moshi sample that is byte-identical to master — this change is {{#jackson}}-guarded and cannot affect moshi output, so it appears pre-existing/flaky. Could a maintainer re-run it?

[seonwooj0810]

The only red check here — Build Kotlin client (samples/client/petstore/kotlin-bigdecimal-default-okhttp4) — is unrelated to this change. It's a transient Maven Central fetch failure, not a code issue:

> Task :compileKotlin FAILED
> Could not resolve com.squareup.moshi:moshi-kotlin:1.15.2
   > Could not GET '.../moshi-kotlin-1.15.2.pom'. Received status code 403 from server: Forbidden

This PR only touches the kotlin Jackson templates/samples — it doesn't touch the okhttp4/moshi sample, and Samples up-to-date passes — so a re-run should clear it. Could a maintainer kick off a re-run when convenient? Thanks!