test: add unit tests for Link model methods

[miguelm-montano]

What does this PR do?

Adds the first unit tests for the Link model, covering all its pure-logic methods. The tests/Unit/ directory was completely empty before this PR.

Methods covered: normalizeTags(), appendParams(), isExpired(), isOverLimit(), hasDeepLinks(), deepLinkFor(), and cacheKey() — 38 tests, 44 assertions

Related issue

Closes #19

Type of change

• Bug fix
• New feature
• Documentation
• Refactor / chore

Checklist

• I ran vendor/bin/pint (code style passes)
• I ran php artisan test (all tests pass) and added/updated tests for my change
• npm run build succeeds (if I touched front-end assets)
• I updated the documentation where relevant
• My change does not introduce a license check, phone-home, or other commercial gating

Notes for reviewers

No production code was modified — only tests/Unit/LinkTest.php was added.

Tests use Tests\TestCase as base class (needed for Eloquent's datetime cast resolution) but do not touch the database, no RefreshDatabase, no factories, no seeders.

A follow-up PR is planned to add feature tests for click tracking edge cases (bot detection, null IP, password-protected links, etc.).