fix: apply alpha to gradient palette colors in data_color()

[taekop]

Summary

GT.data_color(alpha=...) had no effect on cells colored by a gradient palette (the default numeric/factor path). GradientPalette interpolates colors using RGBColor tuples (tuple[int, int, int]) internally, which have no alpha channel, so the alpha= value baked into palette before interpolation was lost by the time each color was assigned to a cell. na_color was unaffected since it goes through _html_color(alpha=...) separately.

This reapplies alpha= to the interpolated colors right after color_scale_fn runs, skipping None entries (which stand in for NA values and get na_color afterward).

Related GitHub Issues and PRs

• Fixes Alpha value gets hidden by data_color() #711

Checklist

• I understand and agree to the Code of Conduct.
• I have followed the Style Guide for Python Code as best as possible for the submitted code.
• I have added pytest unit tests for any new functionality.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.34%. Comparing base (f870830) to head (12e0f4a).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #892   +/-   ##
=======================================
  Coverage   98.33%   98.34%           
=======================================
  Files          52       52           
  Lines        8310     8316    +6     
=======================================
+ Hits         8172     8178    +6     
  Misses        138      138           

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[rich-iannone]

LGTM!