Add tests for PropertyGrid.OnComponentRemoved

[LeafShi1]

Related #12055

Proposed changes

• Add tests for `PropertyGrid.OnComponentRemoved

Microsoft Reviewers: Open in CodeFlow

[Copilot]

Pull request overview

Adds unit test coverage for PropertyGrid’s OnComponentRemoved design-time event handler, improving coverage for removal scenarios related to issue #12055.

Changes:

• Adds new unit tests exercising OnComponentRemoved behavior for null components, single/multi-selection updates, batch mode behavior, and no-op removal.
• Introduces test-only helper component and property tab types to ensure the handler’s code path executes.

Comments suppressed due to low confidence (3)

src/test/unit/System.Windows.Forms/System/Windows/Forms/PropertyGridTests.cs:4593

• This test reaches into the private _selectedObjects field via TestAccessor.Dynamic. The behavior under test is observable via the public SelectedObjects property (which clones the internal array), and using it makes the test less brittle to internal refactors.

        // In batch mode, the component should be removed from internal list
        dynamic testAccessor = propertyGrid.TestAccessor.Dynamic;
        object[] selectedObjects = testAccessor._selectedObjects;

        Assert.Single(selectedObjects);

src/test/unit/System.Windows.Forms/System/Windows/Forms/PropertyGridTests.cs:4648

• TestComponent is already IDisposable via Component, so explicitly implementing IDisposable is redundant. Also, DocumentScopePropertyTab is registered with PropertyTabScope.Component, which makes the name/TabName misleading; rename it to match the actual scope to avoid confusion in future test maintenance.

    [PropertyTab(typeof(DocumentScopePropertyTab), PropertyTabScope.Component)]
    private class TestComponent : Component, IDisposable
    {
        public string TestProperty { get; set; } = "Test";
    }

    private class DocumentScopePropertyTab : PropertyTab
    {
        private Bitmap _bitmap;
        public override string TabName => "Document Tab";
        public override Bitmap Bitmap => _bitmap ??= new(1, 1);

src/test/unit/System.Windows.Forms/System/Windows/Forms/PropertyGridTests.cs:4630

• TestComponent implements IDisposable (via Component) and should be disposed in this test as well to be consistent with the other test cases in this region.

        TestComponent component = new();

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[ricardobossan]

Other than small comments, all LGTM!