Add unit tests for ToolStripAdornerWindowService

[Nora-Zhou01]

Related #10773

Proposed changes

• Add unit test ToolStripAdornerWindowServiceTests.cs for public properties and method of the ToolStripAdornerWindowService
• Enable nullability in ToolStripAdornerWindowServiceTests.cs

Microsoft Reviewers: Open in CodeFlow

[codecov]

Codecov Report

Attention: Patch coverage is 91.53846% with 11 lines in your changes missing coverage. Please review.

Project coverage is 76.06432%. Comparing base (e91e3bc) to head (02c7043).
Report is 325 commits behind head on main.

Additional details and impacted files

@@                 Coverage Diff                 @@
##                main      #13029         +/-   ##
===================================================
+ Coverage   75.75455%   76.06432%   +0.30977%     
===================================================
  Files           3159        3254         +95     
  Lines         635942      643320       +7378     
  Branches       46987       47395        +408     
===================================================
+ Hits          481755      489337       +7582     
+ Misses        150726      150425        -301     
- Partials        3461        3558         +97     

Flag	Coverage Δ
Debug	76.06432% <91.53846%> (+0.30977%)	⬆️
integration	18.01386% <ø> (-0.16092%)	⬇️
production	50.02463% <ø> (+0.70708%)	⬆️
test	96.93532% <91.53846%> (-0.10975%)	⬇️
unit	47.47903% <ø> (+0.94733%)	⬆️

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