[kotlin-spring] Implement skipDefaultValues in mustache templates

[mikhailsnnt]

Summary

• Add skipDefaultValues option to KotlinSpringServerCodegen and wire it into mustache templates
• Wrap default value assignments in dataClassOptVar.mustache and dataClassReqVar.mustache with {{^skipDefaultValues}}...{{/skipDefaultValues}} guards
• When skipDefaultValues=true, generated Kotlin data class properties no longer include = null or other default value assignments

Test plan

• Added testSkipDefaultValues test in KotlinSpringServerCodegenTest that generates petstore models with the flag enabled and asserts = null is absent
• Test passes: mvn test -pl modules/openapi-generator -Dtest=KotlinSpringServerCodegenTest#testSkipDefaultValues

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Summary by cubic

Adds a skipDefaultValues option to KotlinSpringServerCodegen to omit default assignments in generated data classes. When enabled, properties no longer include "= null" or other defaults.

• New Features

  • Added skipDefaultValues switch to KotlinSpringServerCodegen.
  • Updated dataClassOptVar.mustache and dataClassReqVar.mustache to guard default assignments with skipDefaultValues.

• Migration

  • Enable by setting skipDefaultValues=true in additionalProperties or via the generator switch.
  • Default behavior is unchanged when the option is not set.

^{Written for commit 9aecb8f. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 4 files