Cleaned up color_to_rgba template helper

[EvanHahn]

no ref

This makes several fixes to our color_to_rgba template helper:

• Handles more invalid alpha values: non-numbers, non-strings, and NaN.

• Converts everything to TypeScript.

• Uses "color" in all variable names, instead of both "color" and "background color".

• Uses "alpha" in all variable names, instead of both "alpha" and "opacity".

• Adds additional tests.

This shouldn't change anything for folks using the helper correctly. If they were using it incorrectly (e.g., passing an invalid alpha value), we now normalize it a little better. I followed the docs when deciding how things should work.

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ℹ️ Recent review info

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📥 Commits

Reviewing files that changed from the base of the PR and between 5d8ad04 and e987c3d.

📒 Files selected for processing (4)

• ghost/core/core/frontend/helpers/color_to_rgba.js
• ghost/core/core/frontend/helpers/color_to_rgba.ts
• ghost/core/test/unit/frontend/helpers/color-to-rgba.test.js
• ghost/core/test/unit/frontend/helpers/color-to-rgba.test.ts

💤 Files with no reviewable changes (2)

• ghost/core/test/unit/frontend/helpers/color-to-rgba.test.js
• ghost/core/core/frontend/helpers/color_to_rgba.js

🚧 Files skipped from review as they are similar to previous changes (1)

• ghost/core/core/frontend/helpers/color_to_rgba.ts

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Walkthrough

The JavaScript helper for color_to_rgba was removed and replaced by a new TypeScript implementation that parses and sanitizes raw color and alpha inputs, clamps alpha to [0,1], applies the alpha using @tryghost/color-utils, and exports the result. The PR also adds a TypeScript test suite covering valid conversions, alpha parsing/clamping (including infinities and string input), and fallback behavior for invalid colors or alpha values.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately summarizes the main change: converting and cleaning up the color_to_rgba helper, which aligns with the TypeScript conversion and bug fixes across the changeset.
Description check	✅ Passed	The description directly explains the changes made to the color_to_rgba helper, including TypeScript conversion, improved alpha handling, naming standardization, and additional tests.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch color_to_rgba-cleanups

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