Added unit tests for low-coverage System.Windows.Forms classes

[Sathish-087]

Fixes #13442

Proposed changes

• Added 9 unit tests for RadioButtonFlatAdapter.
• Added 11 unit tests for RadioButtonPopupAdapter.
• Added 8 unit tests for ToolStrip.RestoreFocusMessageFilter.
• Added 8 unit tests for HtmlHistory.
• Covered paint operations (PaintUp, PaintDown, PaintOver) for radio button adapter implementations.
• Added coverage for adapter creation and layout behavior validation.
• Added tests for DPI-aware check size calculations and shadowed text behavior.
• Added coverage for checked-state rendering and text/image rendering scenarios.
• Added tests validating focus restoration and message filtering behavior in ToolStrip.RestoreFocusMessageFilter.
• Added tests covering HtmlHistory navigation APIs (Back, Forward, Go), property accessors, argument validation, and disposal behavior.
• Improved unit test coverage for low-coverage classes in the System.Windows.Forms namespace.

Customer Impact

• Improves unit test coverage for several previously low-coverage System.Windows.Forms classes.
• Increases confidence in rendering, layout, focus restoration, and HTML history navigation functionality.
• Helps prevent regressions in public and protected virtual APIs across the tested components.

Regression?

• No

Risk

• Low (tests only, no product code changes)

Test methodology

• Executed existing WinForms unit test suite.
• Added and executed new unit tests for RadioButtonFlatAdapter.
• Added and executed new unit tests for RadioButtonPopupAdapter.
• Added and executed new unit tests for ToolStrip.RestoreFocusMessageFilter.
• Added and executed new unit tests for HtmlHistory.

Accessibility testing

NA

Test environment(s)

• Visual Studio (Test Explorer)
• Windows
• .NET 11

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