Adds coverage to FormatStringEditor

Related #10773

## Proposed changes

- Adds code coverage to `FormatStringEditor`

## Customer Impact

- None

## Regression?

- No

## Risk

- Minimal

## Test methodology

- Unit tests

## Test environment(s)

- 9.0.100-rc.1.24452.12

src/System.Windows.Forms.Design/tests/UnitTests/System/Windows/Forms/Design/FormatStringEditorTests.cs

@@ -0,0 +1,100 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+#nullable enable
+
+using System.ComponentModel;
+using System.ComponentModel.Design;
+using System.Drawing.Design;
+using System.Windows.Forms.TestUtilities;
+using Moq;
+
+namespace System.Windows.Forms.Design.Tests;
+
+public class FormatStringEditorTests
+{
+    private readonly FormatStringEditor _editor = new();
+    private readonly Mock<IWindowsFormsEditorService> _mockEditorService;
+    private readonly Mock<IComponentChangeService> _mockChangeService;
+    private readonly Mock<IServiceProvider> _provider;
+    private readonly Mock<ITypeDescriptorContext> _context;
+    private readonly DataGridViewCellStyle _cellStyle;
+
+    public FormatStringEditorTests()
+    {
+        _mockEditorService = new();
+        _mockChangeService = new();
+        _cellStyle = new() { Format = "Initial" };
+
+        _provider = new(MockBehavior.Strict);
+        _provider.Setup(p => p.GetService(typeof(IWindowsFormsEditorService))).Returns(_mockEditorService.Object);
+        _provider.Setup(p => p.GetService(typeof(IComponentChangeService))).Returns(_mockChangeService.Object);
+
+        _context = new(MockBehavior.Strict);
+        _context.Setup(c => c.Instance).Returns(_cellStyle);
+    }
+
+    [Theory]
+    [CommonMemberData(typeof(CommonTestHelperEx), nameof(CommonTestHelperEx.GetEditValueInvalidProviderTestData))]
+    public void EditValue_InvalidProvider_ReturnsValue(IServiceProvider provider, object value)
+    {
+        _editor.EditValue(null, provider, value).Should().Be(value);
+    }
+
+    [Theory]
+    [CommonMemberData(typeof(CommonTestHelperEx), nameof(CommonTestHelperEx.GetITypeDescriptorContextTestData))]
+    public void GetEditStyle_Invoke_ReturnsModal(ITypeDescriptorContext context)
+    {
+        _editor.GetEditStyle(context).Should().Be(UITypeEditorEditStyle.Modal);
+    }
+
+    [WinFormsFact]
+    public void EditValue_ShowsDialogAndUpdatesValue()
+    {
+        object? result = _editor.EditValue(_context.Object, _provider.Object, "NewValue");
+
+        result.Should().Be("NewValue");
+        _mockEditorService.Verify(es => es.ShowDialog(It.IsAny<FormatStringDialog>()), Times.Once);
+    }
+
+    [WinFormsFact]
+    public void EditValue_CallsOnComponentChanging_Once()
+    {
+        _editor.EditValue(_context.Object, _provider.Object, "NewValue");
+
+        _mockChangeService.Verify(
+            cs => cs.OnComponentChanging(_cellStyle, TypeDescriptor.GetProperties(_cellStyle)["Format"]),
+            Times.Once);
+
+        _mockChangeService.Verify(
+            cs => cs.OnComponentChanging(_cellStyle, TypeDescriptor.GetProperties(_cellStyle)["NullValue"]),
+            Times.Once);
+
+        _mockChangeService.Verify(
+            cs => cs.OnComponentChanging(_cellStyle, TypeDescriptor.GetProperties(_cellStyle)["FormatProvider"]),
+            Times.Once);
+    }
+
+    [WinFormsFact]
+    public void EditValue_WithDirtyDialog_CallsOnComponentChanged_Once()
+    {
+        using FormatStringDialog dialog = new(_context.Object);
+
+        _editor.TestAccessor().Dynamic._formatStringDialog = dialog;
+        dialog.TestAccessor().Dynamic._dirty = true;
+
+        _editor.EditValue(_context.Object, _provider.Object, "NewValue");
+
+        _mockChangeService.Verify(
+            cs => cs.OnComponentChanged(_cellStyle, TypeDescriptor.GetProperties(_cellStyle)["Format"], null, null),
+            Times.Once);
+
+        _mockChangeService.Verify(
+            cs => cs.OnComponentChanged(_cellStyle, TypeDescriptor.GetProperties(_cellStyle)["NullValue"], null, null),
+            Times.Once);
+
+        _mockChangeService.Verify(
+            cs => cs.OnComponentChanged(_cellStyle, TypeDescriptor.GetProperties(_cellStyle)["FormatProvider"], null, null),
+            Times.Once);
+    }
+}