[BUG][SWIFT6] unalias additionalProperties before building map type declaration

[winrid]

Closes #23666.

A nested map property whose additionalProperties is a $ref to an aliased map schema produced [String: Dictionary] (no generic args, doesn't compile in Swift 6) instead of the resolved [String: [String: String]].

StringMap:
  type: object
  additionalProperties: { type: string }
EmailTemplate:
  properties:
    translationOverridesByLocale:
      type: object
      additionalProperties:
        $ref: '#/components/schemas/StringMap'

Fix

Wrap ModelUtils.getAdditionalProperties(p) with unaliasSchema(...) in Swift6ClientCodegen.getTypeDeclaration so the alias resolves to the actual schema before recursion.

Test

New nestedMapRefTypeTest in Swift6ClientCodegenModelTest against swift6_nested_map.yaml asserts the resolved type. Full swift6 suite (39 tests) passes.

PR checklist

• Read the contribution guidelines.
• Pull Request title clearly describes the work in the pull request and Pull Request description provides details about how to validate the work. Missing information here may result in delayed response from the community.
• Run the following to build the project and update samples:

  ./mvnw clean package
  

• File the PR against the correct branch: master (5.x.x) (patches will be cascaded from master to other branches by the maintainers)
• If your PR is targeting a particular programming language, @mention the technical committee members, so they are more likely to review the pull request.

cc @4brunu @yutailang0119

[cubic-dev-ai]

No issues found across 3 files