fix(kotlin-spring): preserve 'default' response code in postProcessOperationsWithModels

[mdemblani]

Description

Fixes #17445

When an OpenAPI spec defines a default response (a catch-all with no specific HTTP status code), the generator internally represents it with code = "0". The existing logic in KotlinSpringServerCodegen.postProcessOperationsWithModels unconditionally mapped "0" → "200", causing every generated @ApiResponse annotation for default responses to incorrectly use responseCode = "200" instead of responseCode = "default".

Root cause

// Before (line 1607)
if ("0".equals(resp.code)) {
    resp.code = "200";  // wrong for isDefault responses
}

Fix

Check resp.isDefault before mapping — only fall back to "200" for non-default zero-coded responses:

// After
if ("0".equals(resp.code)) {
    resp.code = resp.isDefault ? "default" : "200";
}

This mirrors the fix applied to SpringCodegen (Java) in #14399.

Generated code: before vs after

Before:

@ApiResponse(responseCode = "200", description = "Unexpected error", content = [Content(schema = Schema(implementation = Error::class))])

After:

@ApiResponse(responseCode = "default", description = "Unexpected error", content = [Content(schema = Schema(implementation = Error::class))])

Checklist

• The fix is consistent with the approach taken in [Java][Spring] fix default response code #14399 for SpringCodegen
• Change is minimal and surgical — one line in postProcessOperationsWithModels
• Does not affect responses with explicit HTTP status codes

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Summary by cubic

Preserves the OpenAPI default response in the Kotlin Spring generator without mutating response codes. @ApiResponse now renders responseCode = "default" while numeric codes are kept elsewhere to avoid crashes with useResponseEntity = false.

• Bug Fixes
  • Keep resp.code numeric in KotlinSpringServerCodegen.postProcessOperationsWithModels; emit "default" in swagger2 annotations via {{#isDefault}} in api.mustache and apiInterface.mustache.
  • No changes needed to SpringHttpStatusLambda or swagger1 templates; they continue using numeric codes.
  • Add defaultResponseCodeRenderedAsDefault test to verify "default" is rendered, explicit 200 stays numeric, and "0" never appears.
  • Regenerated Kotlin Spring petstore samples.

^{Written for commit aabbc52. Summary will update on new commits. Review in cubic}

[cubic-dev-ai]

No issues found across 1 file

_{Re-trigger cubic}

[Copilot]

Pull request overview

This PR aims to fix the Kotlin Spring server generator so OpenAPI default responses are emitted as @ApiResponse(responseCode = "default") instead of incorrectly being rewritten to "200" during postProcessOperationsWithModels.

Changes:

• Update KotlinSpringServerCodegen.postProcessOperationsWithModels to map internal response code == "0" to "default" when resp.isDefault is true, otherwise keep the "200" fallback.

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[wing328]

please follow step 3 to update the samples: https://github.com/OpenAPITools/openapi-generator/actions/runs/26193287517/job/77142730074?pr=23833