Convert Label to discriminated union

[kharus]

No description provided.

[Copilot]

Pull Request Overview

This PR converts the Label implementation from subclassing (PreLabel and PrePostLabel) to a discriminated union, streamlining label creation and usage across the model and tests.

• Replaces abstract Label and its subclasses with a union type for labels
• Updates the deserialization and encoding functions to handle the new structure
• Revises unit tests to use the discriminated union syntax

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
src/model/Item.ts	Removed classes and created a union type for Label; updated label deserialization and encoding logic.
src/model/tests/Item.test.ts	Modified tests to use the new Label union values.
src/model/tests/Interview.test.ts	Updated label instantiation to use the union format in tests.
src/model/tests/Relevance.test.ts	Replaced PreLabel usage with the union type for Label.

Comments suppressed due to low confidence (1)

src/model/Item.ts:98

• Consider updating the error message to include the full JSON payload (e.g., using JSON.stringify(json)) to provide more context when deserialization fails.

throw new Error(`Unknown label type: ${json.type}`);