[kotlin-client][jvm-ktor] Support nullable response types

[TheJJ]

The jvm-ktor library did not handle OpenAPI schemas that allow null as a valid response body (e.g. anyOf: [T, {type: null}] or allOf: [T] with nullable: true).

Two template bugs prevented this from working:

1. api.mustache used {{{returnType}}} directly, ignoring the isNullable flag on the response property. Now it appends ? when returnProperty.isNullable is true.

2. HttpResponse.kt.mustache hardcoded <T : Any> on every generic type parameter. This made HttpResponse<SomeType?> invalid at the Kotlin type-system level. Removing the : Any constraint lets the generated code use nullable types naturally.

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Summary by cubic

Enables nullable response bodies in the Kotlin jvm-ktor generator so clients can return HttpResponse<T?> when an OpenAPI schema allows null (e.g., anyOf [T, null] or nullable: true). Generated code now compiles and handles nulls correctly.

• Bug Fixes
  • api.mustache now appends ? to the return type when returnProperty.isNullable is true.
  • Removed : Any bounds from generics in HttpResponse.kt.mustache (including HttpResponse, BodyProvider, TypedBodyProvider, MappedBodyProvider, wrap, and typedBody/map) to allow nullable types.

^{Written for commit df3caf7. Summary will update on new commits. Review in cubic}

[cubic-dev-ai]

No issues found across 2 files

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