test(lib): add coverage for propertyService filters and id lookup

[Dataguru-tech]

Summary
Adds unit tests for propertyService.ts (the property data layer), which previously had no coverage.

What's covered

• searchProperties: filtering by property type, price range, location, and combined filters
• searchProperties: empty-result case when no property matches the filters
• getPropertyById: valid id lookup and missing id (returns null)

Redis, IndexedDB, and network-status checks are mocked so tests exercise the actual filter/sort/lookup logic against MOCK_PROPERTIES rather than the caching layer. This pins the current filter contract so the mock-to-real-API migration (#13) can't silently change listing behavior.

Out of scope
The properties API mock (#13) and the API route — per the issue.
Closes #929

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[github-actions]

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