refactor(json-difference): extract sentinel unwrap helper

[lukascivil]

Objective

The sentinel strings '@{}' and '@[]' were duplicated across three files (get-edited-paths.ts, get-paths-diff.ts, sanitize-delta.ts). This refactor extracts them into a single unwrap-sentinel helper to eliminate the duplication and make future sentinel changes easier to apply consistently.

What was done

• Added libs/json-difference/src/helpers/unwrap-sentinel.ts with OBJECT_SENTINEL, ARRAY_SENTINEL constants and an unwrapSentinel function that maps sentinel values back to {} / []
• Added unit tests for unwrapSentinel in libs/json-difference/src/helpers/unwrap-sentinel.spec.ts
• Replaced inline sentinel comparisons in get-edited-paths.ts, get-paths-diff.ts, and sanitize-delta.ts with the new helper; sanitize-delta.ts now preserves the original tuple reference when no sentinel is present
• Exported unwrap-sentinel from libs/json-difference/src/helpers/index.ts
• Added tsconfig.spec.json reference to libs/json-difference/tsconfig.json
• Added JSON fixture files (fixture/oldJson.json, fixture/newJson.json, fixture/delta.json) and a fixture-based integration test in get-diff.spec.ts that verifies the full diff output against a known snapshot

Test plan

• Run yarn nx test json-difference — all tests should pass including the new fixture test
• Run yarn nx lint json-difference — no warnings