Make kind property readonly in ReferenceListSource

[Copilot]

The kind property on ReferenceListSource was mutable, allowing accidental reassignment post-construction.

Change

• Added readonly modifier to kind in ReferenceListSource

// before
kind = SourceKind.ReferenceList;

// after
readonly kind = SourceKind.ReferenceList;

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[Copilot]

Pull request overview

This PR enhances type safety by making the kind property readonly in the ReferenceListSource class, preventing accidental reassignment after construction. The change aligns with TypeScript best practices for immutable discriminator properties.

Changes:

• Added readonly modifier to the kind property in ReferenceListSource class

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[Copilot]

While adding readonly to the kind property is a good practice, this creates an inconsistency with the other source classes in the codebase. The DocumentSource (document.ts:4), ListSource (list.ts:16), and ReferenceDocumentSource (reference-document.ts:4) classes all declare kind without the readonly modifier. Consider applying this change to all source classes for consistency, or document why only ReferenceListSource requires this protection.

Suggested change

	readonly kind = SourceKind.ReferenceList;
	kind = SourceKind.ReferenceList;