Add unit tests for cache dependencies

[WarLikeLaux]

Q	A
Is bugfix?	❌
New feature?	❌
Breaks BC?	❌
Fixed issues

What does this PR do?

Add unit tests for ChainedDependency, ExpressionDependency, and DummyCache.

Before / After

File	Tests	Methods	Lines	MSI
ChainedDependency.php	0 → 15	0/3 → 2/3	0/9 (0%) → 8/9 (89%)	— → 100% (18/18)
ExpressionDependency.php	0 → 13	0/1 → 1/1	0/1 (0%) → 1/1 (100%)	— → 100% (1/1)
DummyCache.php	0 → 18	0/5 → 5/5	0/5 (0%) → 5/5 (100%)	— → 100% (10/10)

46 tests, 70 assertions total.

ChainedDependency uncovered line 58: generateDependencyData() returns null. Dead code - the method is never called because ChainedDependency overrides both evaluateDependency() and isChanged(). Exists only to satisfy the abstract contract from Dependency.

Changelog not required, because no source files changed.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.21%. Comparing base (37b6e7b) to head (dd4d201).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #20775      +/-   ##
============================================
+ Coverage     80.13%   80.21%   +0.07%     
  Complexity    11533    11533              
============================================
  Files           374      374              
  Lines         30206    30206              
============================================
+ Hits          24207    24230      +23     
+ Misses         5999     5976      -23     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.