Makes PropertyGrid for CoboBox show "(none)" when DataSource is null …

[ricardobossan]

Fixes #12570

Root Cause

The ComboBox.DataSource property was missing a TypeConverter to handle null value display in PropertyGrid. When DataSource is null, PropertyGrid shows empty/blank instead of "(none)" text, breaking consistency with .NET Framework behavior and other similar properties like DisplayMember and ValueMember.

Proposed changes

• Created DataSourceConverter class that inherits from ReferenceConverter and converts null values to "(none)" for string display
• Applied TypeConverter attribute to ComboBox.DataSource property to use the new converter
• Added comprehensive unit tests to verify converter behavior across different cultures and scenarios

Customer Impact

Restores .NET Framework parity for PropertyGrid display. Developers using PropertyGrid to inspect ComboBox properties will now see consistent "(none)" text when DataSource is null, improving design-time experience.

Regression?

No. This is a missing feature/behavior that existed in .NET Framework but was lost in .NET Core migration.

Risk

Minimal.

Screenshots

Before

After

Test methodology

• Unit tests for DataSourceConverter covering null conversion, culture handling, and edge cases
• Manual testing with PropertyGrid and ComboBox to verify "(none)" display

Test environment(s)

• 10.0.100-alpha.1.24573.1

Microsoft Reviewers: Open in CodeFlow

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 77.11928%. Comparing base (8f9c849) to head (c99c9b8).
⚠️ Report is 28 commits behind head on main.

Additional details and impacted files

@@                 Coverage Diff                 @@
##                main      #13860         +/-   ##
===================================================
- Coverage   77.12127%   77.11928%   -0.00199%     
===================================================
  Files           3273        3275          +2     
  Lines         644931      644936          +5     
  Branches       47692       47695          +3     
===================================================
- Hits          497379      497370          -9     
- Misses        143876      143892         +16     
+ Partials        3676        3674          -2     

Flag	Coverage Δ
Debug	77.11928% <100.00000%> (-0.00199%)	⬇️
integration	18.99332% <0.00000%> (+0.00995%)	⬆️
production	51.95209% <100.00000%> (-0.00537%)	⬇️
test	97.41261% <100.00000%> (+0.00005%)	⬆️
unit	49.36262% <100.00000%> (-0.01304%)	⬇️

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