Fix MessageBox not setting SchemeName correctly (#4567)

[tig]

Summary

• Fixes MessageBox not setting SchemeName property correctly
• Added unit tests to verify Query uses "Dialog" scheme and ErrorQuery uses "Error" scheme

Changes

• MessageBox.cs: Uncommented the SchemeName assignment that correctly sets the scheme based on whether it's an error dialog
• MessageBoxTests.cs: Added two new unit tests:
  • Query_Sets_Dialog_SchemeName: Verifies MessageBox.Query uses the Dialog color scheme
  • ErrorQuery_Sets_Error_SchemeName: Verifies MessageBox.ErrorQuery uses the Error color scheme

Test plan

• Added unit tests that fail before the fix and pass after
• Verified all existing MessageBox tests still pass (5 tests total)
• Tests verify the correct SchemeName is set for both Query and ErrorQuery

Related

Fixes #4567

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 77.65%. Comparing base (23ee30b) to head (f347f88).
⚠️ Report is 1 commits behind head on v2_develop.

Additional details and impacted files

@@              Coverage Diff               @@
##           v2_develop    #4568      +/-   ##
==============================================
+ Coverage       77.62%   77.65%   +0.02%     
==============================================
  Files             427      427              
  Lines           45364    45362       -2     
  Branches         6400     6400              
==============================================
+ Hits            35216    35226      +10     
+ Misses           8319     8306      -13     
- Partials         1829     1830       +1     

Files with missing lines	Coverage Δ
Terminal.Gui/Views/Dialog.cs	94.49% <100.00%> (-0.06%)	⬇️

... and 4 files with indirect coverage changes

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