Add unit test for MaskedTextBoxDesignerActionList

[Syareel-Sukeri]

Related #10773

Proposed changes

1. Add unit test MaskedTextBoxDesignerActionListTests.cs for public properties and method of the MaskedTextBoxDesignerActionList.
2. Enable nullability in MaskedTextBoxDesignerActionListTests.cs.

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[codecov]

Codecov Report

Attention: Patch coverage is 95.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 75.77638%. Comparing base (65f897b) to head (8382481).
Report is 46 commits behind head on main.

Additional details and impacted files

@@                 Coverage Diff                 @@
##                main      #12491         +/-   ##
===================================================
+ Coverage   75.65407%   75.77638%   +0.12230%     
===================================================
  Files           3150        3154          +4     
  Lines         635831      635867         +36     
  Branches       47020       46978         -42     
===================================================
+ Hits          481032      481837        +805     
+ Misses        151352      150594        -758     
+ Partials        3447        3436         -11     

Flag	Coverage Δ
Debug	75.77638% <95.00000%> (+0.12230%)	⬆️
integration	18.26795% <ø> (+0.01887%)	⬆️
production	49.39852% <ø> (+0.20019%)	⬆️
test	97.05072% <95.00000%> (+0.02048%)	⬆️
unit	46.37650% <ø> (+0.22053%)	⬆️

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[ricardobossan]

Since these fields are explicitly defined, their initialization could be simplified by just calling new().

[ricardobossan]

It seems that expectedMask will always end up as an empty string, even though it’s initially assigned the value "000-00-0000". This happens because resultMask evaluates to an empty string, and the fallback on line 73 (expectedMask = resultMask ?? expectedMask) causes expectedMask to be overridden.

Additionally, _maskedTextBox.Mask starts as an empty string before SetMask() is called and remains unchanged afterward. While the test passes, it doesn’t validate whether SetMask() updates _maskedTextBox.Mask as intended. The test, in its current form, doesn’t seem to verify meaningful behavior.

Before SetMask() is called:

After SetMask() is called: