Fix #20304 : preserve sibling properties on $ref

[krumbs8749]

Core changes

• Fixed [BUG] [OpenAPI 3.1] siblings of $ref are lost #20304
• Ensure all ref sibling properties from OAS 3.1 are not lost after resolving schema.
• This PR creates mergeSiblingFields() function (in ModelUtils) to copy every non-null OAS 3.1 sibling keyword from the wrapper onto the resolved definition, and created deepCopy() helper to preserve concrete schema subtypes (so flags like isNull/isDate still work).

Added tests:

• A comprehensive smoke test (DefaultCodegenTest.testRefSiblingMergingOnAllAliasForms) was added to assert that every allowed sibling property is carried through on a variety of alias forms ($ref, allOf, free-form object, array alias, map alias, primitive alias).

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[wing328]

thanks for the pr

when you've time, please resolve the merge conflicts and merge the latest master into your branch (that should fix the rust server CI failure)