Add contrast color regression coverage

[9larsons]

Summary

Adds five independent regression cases for textColorForBackgroundColor following #1031. This is a tests-only change: all existing assertions remain intact and no production or generated files change.

Coverage

• Adds the three exact light backgrounds reported in {{contrast_text_color}} helper returns incorrect text color Ghost#27797: #dacafe, #ffa5b1, and #a3e6ff.
• Adds #b9b9b9 and #bbbbbb to cover the white/black decision immediately below and above the YIQ threshold.
• Keeps the existing #cccccc RGB blue-channel regression unchanged.

A deliberate check using the previous Color.b() calculation fails four of the five added cases, confirming the new cases protect the visual correction in #1031.

Validation

• yarn workspace @tryghost/color-utils build
• yarn workspace @tryghost/color-utils test — 16 tests passed, 100% coverage
• yarn test — SDK-wide lint and tests passed across all 17 projects
• git diff --check

Refs #1031 and TryGhost/Ghost#27797.

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[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (07144be) to head (4ae537d).

Additional details and impacted files

@@            Coverage Diff            @@
##              main     #1049   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           30        30           
  Lines         3060      3060           
  Branches       459       459           
=========================================
  Hits          3060      3060           

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