Improve ButtonDarkModeAdapter color handling and remove inappropriate HighContrast logic

[LeafShi1]

Fixes #14155

Proposed changes

• Preserve the core logic that prevents background color changes on hover when UseVisualStyleBackColor is false or custom colors are set
• Deleted IsHighContrastHighlighted() checks from both GetButtonTextColor and GetButtonBackColor methods, allowing the adapter to focus solely on Dark Mode rendering
• Replace color comparisons with Control.ShouldSerializeBackColor() and Control.ShouldSerializeForeColor() to accurately detect when users have explicitly set color properties

Customer Impact

• When UseVisualStyleBackColor is set to false, buttons now maintain a consistent background color and no longer change appearance on hover/press states.

Regression?

• Yes

Risk

• Minimal

Screenshots

Before

DarkModeResults.mp4

After

AfterChanges.mp4

Test methodology

• Manually

Test environment(s)

• .net 11.0.0-alpha.1.25617.103

Microsoft Reviewers: Open in CodeFlow

[codecov]

Codecov Report

❌ Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 77.15415%. Comparing base (01eefd4) to head (a785abe).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@                 Coverage Diff                 @@
##                main      #14161         +/-   ##
===================================================
+ Coverage   77.15180%   77.15415%   +0.00235%     
===================================================
  Files           3279        3279                 
  Lines         645333      645325          -8     
  Branches       47720       47718          -2     
===================================================
+ Hits          497886      497895          +9     
+ Misses        143754      143735         -19     
- Partials        3693        3695          +2     

Flag	Coverage Δ
Debug	77.15415% <0.00000%> (+0.00235%)	⬆️
integration	18.98825% <0.00000%> (+0.00818%)	⬆️
production	52.01864% <0.00000%> (+0.00457%)	⬆️
test	97.40559% <ø> (ø)
unit	49.46638% <0.00000%> (-0.00107%)	⬇️

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