Add code coverage for DesignerUtils

[Zheng-Li01]

Related #10773

Proposed changes

Add unit test file: DesignerUtilsTests.cs for public methods & properties of the DesignerUtils.cs

Note: working on another PR for the remaining test case

Microsoft Reviewers: Open in CodeFlow

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 62.62927%. Comparing base (99b335c) to head (45a7b9c).
Report is 14 commits behind head on main.

Additional details and impacted files

@@                 Coverage Diff                  @@
##                main      #13341          +/-   ##
====================================================
- Coverage   95.68899%   62.62927%   -33.05973%     
====================================================
  Files            542        1561        +1019     
  Lines          63419      159839       +96420     
  Branches        1451       14915       +13464     
====================================================
+ Hits           60685      100106       +39421     
- Misses          2400       58958       +56558     
- Partials         334         775         +441     

Flag	Coverage Δ
Debug	62.62927% <100.00000%> (-33.05973%)	⬇️
integration	11.32692% <ø> (?)
production	40.81120% <ø> (?)
test	95.69857% <100.00000%> (+0.00957%)	⬆️
unit	38.21001% <ø> (?)

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Pull Request Overview

This PR adds unit tests to verify the public methods and properties of DesignerUtils as part of enhancing code coverage related to issue #10773.

• Added a new test file (DesignerUtilsTests.cs) containing tests for BoxImage, HoverBrush, MinDragSize, and drawing methods.
• Introduced theory tests to validate behavior of the DrawResizeBorder method based on different background colors.

[Tanya-Solyanik]

Thank you!