fix(core): SpecFilter follows ApiResponse $ref to avoid pruning referenced schemas

[kuntal1461]

Fixes #4960

Pull Request

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Description

Bug fix: SpecFilter.removeBrokenReferenceDefinitions did not follow ApiResponse.$ref, so schemas reachable only through #/components/responses/* were considered unreferenced and pruned.
This PR teaches the pruning pass to resolve ApiResponse.$ref and traverse the resolved response's headers and content to collect schema refs, ensuring those schemas are retained.

What changed (minimal patch):

• In addApiResponseSchemaRef(...), add a $ref branch:
  • Resolve simple name via RefUtils.extractSimpleName(ref).getLeft()
  • Mark the referenced ApiResponse component as used
  • Traverse resolved headers and content to collect schema refs
• No signature changes to existing helper methods; no behavioral changes outside pruning.

Fixes: #4960

Type of Change

• 🐛 Bug fix

Checklist

• The PR title is descriptive
• The code builds and passes tests locally (mvn clean install)
• I have added/updated tests as needed (see Test Plan below)
• I have added/updated documentation where applicable
• I have linked related issues (if any)

Test Plan

Manual verification:

1. Define an operation with:

   @ApiResponse(responseCode = "404", ref = "#/components/responses/MyCustomSchema")