[Kotlin] Fix explicitApi/nonPublicApi/generateOneOfAnyOfWrappers generation for kotlinx_serialization

[yzaoui]

There are currently issues with the options explicitApi, nonPublicApi, and generateOneOfAnyOfWrappers options when used together with kotlinx_serialization

1. explicitApi + generateOneOfAnyOfWrappers: This generates a wrapper which is missing the public keyword inside of the value class' val value property, which makes the library fail to compile when explicitApi() is enabled in the library since the property doesn't have an explicit public
2. nonPublicApi + generateOneOfAnyOfWrappers: I haven't run into this myself, but while fixing the above I noticed that the template attempts to insert internal in the subclass of the internal interface, but Kotlin doesn't allow this, so it generates invalid code

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@karismann @Zomzog @andrewemery @4brunu @stefankoppier @e5l @dennisameling @wing328

---

Summary by cubic

Fixes Kotlin kotlinx_serialization generation of oneOf/anyOf wrapper value classes so projects compile with explicitApi and remain valid with nonPublicApi. Adjusts visibility on wrappers and their value properties to avoid invalid code.

• Bug Fixes
  • For explicitApi + generateOneOfAnyOfWrappers: make val value explicitly public.
  • For nonPublicApi + generateOneOfAnyOfWrappers: stop marking the nested value class as internal; make only the constructor value property internal.
  • Adds tests to verify both modes with kotlinx_serialization.

^{Written for commit d39a441. Summary will update on new commits. Review in cubic}

[yzaoui]

{{#nonPublicApi}}internal {{/nonPublicApi}}{{^nonPublicApi}}{{#explicitApi}}public {{/explicitApi}}{{/nonPublicApi}}

becomes

{{#explicitApi}}public {{/explicitApi}}

because if nonPublicApi is true, this is an internal interface, which can never have an internal subclass. This subclass is thus effectively always public

[cubic-dev-ai]

No issues found across 3 files

_{Re-trigger cubic}

[yzaoui]

The second change is

val value

becoming

{{#nonPublicApi}}internal {{/nonPublicApi}}{{^nonPublicApi}}{{#explicitApi}}public {{/explicitApi}}{{/nonPublicApi}}val value

Adding the missing public in explicit API mode, and adding the missing internal in non-public mode