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 76.58686%. Comparing base (71a8e95) to head (39a5a37).
Report is 132 commits behind head on main.

Additional details and impacted files

@@                 Coverage Diff                 @@
##                main      #13435         +/-   ##
===================================================
- Coverage   76.60371%   76.58686%   -0.01686%     
===================================================
  Files           3230        3253         +23     
  Lines         639097      641110       +2013     
  Branches       47289       47442        +153     
===================================================
+ Hits          489572      491006       +1434     
- Misses        145953      146450        +497     
- Partials        3572        3654         +82     

Flag	Coverage Δ
Debug	76.58686% <100.00000%> (-0.01686%)	⬇️
integration	18.85583% <ø> (+0.06775%)	⬆️
production	51.01200% <ø> (+0.00097%)	⬆️
test	97.41258% <100.00000%> (+0.00847%)	⬆️
unit	48.45871% <ø> (+0.03964%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

Copilot had a point

[ricardobossan]

Thank you!

[Copilot]

Pull Request Overview

This PR adds extensive unit tests to increase code coverage for public methods and properties in DesignerUtils.

• Introduces tests covering theme styles application, container checks, generic type filtering, unique site name generation, bounds calculation, and text baseline computation.
• Validates behavior for both expected results and exception throwing conditions.

Comments suppressed due to low confidence (1)

src/System.Windows.Forms.Design/tests/UnitTests/System/Windows/Forms/Design/DesignerUtilsTests.cs:540

• The test method name 'GetTextBaseline_ShouldThrowNullNullException_WhenControlIsNull' contains a probable typo. Consider renaming it to 'GetTextBaseline_ShouldThrowNullReferenceException_WhenControlIsNull' to accurately reflect the expected exception.

public void GetTextBaseline_ShouldThrowNullNullException_WhenControlIsNull()

[ricardobossan]

All LGTM, except that the CI fails no matter how many times I attempt to re-run failed checks. What do you think @LeafShi1 @Epica3055 ?

Nevermind. The issues were addressed.