[DarkMode] Fix MDIParent MainMenuStrip System Bitmaps

[memoarfaa]

Fixes #13475

Proposed changes

• Adjust bitmap color transformation logic in ToolStripMenuItem.cs to render MainMenuStrip bitmaps correctly in dark mode.
• Apply color matrix transformation to invert and adjust bitmap colors for dark mode in MDIParent scenarios.
• Ensure icons visually match the dark theme when Application.SetColorMode(SystemColorMode.Dark) is enabled.

Customer Impact

• Resolves an issue where MainMenuStrip bitmaps in MDIParent forms were drawn with light colors in dark mode.
• Improves UI consistency for users utilizing dark mode in their WinForms applications.

Regression?

• No

Risk

• Low: Changes are isolated to bitmap rendering for dark mode and do not affect standard rendering paths.

Screenshots

Before

After

Test methodology

• Manual testing in a sample MDI WinForms .NET Core project.
• Verified that MainMenuStrip bitmaps render correctly in both light and dark modes.
• Confirmed no regressions in standard (light mode) theme rendering.

Accessibility testing

Test environment(s)

• .NET 10.0.100-preview5.25266.103
• Windows 11 x64

Microsoft Reviewers: Open in CodeFlow

[codecov]

Codecov Report

Attention: Patch coverage is 8.33333% with 22 lines in your changes missing coverage. Please review.

Project coverage is 76.59986%. Comparing base (92e2dcc) to head (d40c89c).
Report is 4 commits behind head on main.

Additional details and impacted files

@@                 Coverage Diff                 @@
##                main      #13480         +/-   ##
===================================================
- Coverage   76.60590%   76.59986%   -0.00604%     
===================================================
  Files           3230        3230                 
  Lines         639097      639180         +83     
  Branches       47289       47297          +8     
===================================================
+ Hits          489586      489611         +25     
- Misses        145945      146000         +55     
- Partials        3566        3569          +3     

Flag	Coverage Δ
Debug	76.59986% <8.33333%> (-0.00604%)	⬇️
integration	18.78577% <0.00000%> (-0.00127%)	⬇️
production	51.00986% <8.33333%> (-0.00605%)	⬇️
test	97.40411% <ø> (ø)
unit	48.39842% <8.33333%> (+0.00308%)	⬆️

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