Mdemblani fix/kotlin spring default response code

[wing328]

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Summary by cubic

Fixes incorrect rendering of OpenAPI default responses in Kotlin Spring by emitting responseCode="default" in Swagger v2 @ApiResponse. This corrects generated docs and prevents crashes when useResponseEntity=false.

• Bug Fixes
  • Update api.mustache and apiInterface.mustache to render responseCode = "default" when isDefault is true; keep numeric codes otherwise.
  • Add regression test to verify "default" is emitted, explicit 200s remain, and "0" never appears; also confirms no crash with useResponseEntity=false.
  • Regenerate Kotlin Spring Petstore samples to reflect the fix.
  • Addresses [Kotlin][Spring] Response code 200 generated for default responses #17445.

^{Written for commit 4f48003. Summary will update on new commits.}