refactor(securestoremanager): add platform service abstractions and constants

Implement deferred code review findings:
- Add IDialogService/IClipboardService interfaces for testable platform operations
- Create AvaloniaDialogService and AvaloniaClipboardService implementations
- Extract dialog strings and file extensions to centralized Constants classes
- Refactor ViewModels to use DI instead of event delegates
- Update tests to use mock services

NEW/tests/JdeScoping.SecureStoreManager.Tests/ViewModels/SecretItemViewModelTests.cs

@@ -1,16 +1,25 @@
+using NSubstitute;
 using Shouldly;
 using Xunit;
+using JdeScoping.SecureStoreManager.Services;
 using JdeScoping.SecureStoreManager.ViewModels;
 
 namespace JdeScoping.SecureStoreManager.Tests.ViewModels;
 
 public class SecretItemViewModelTests
 {
+    private readonly IClipboardService _mockClipboardService;
+
+    public SecretItemViewModelTests()
+    {
+        _mockClipboardService = Substitute.For<IClipboardService>();
+    }
+
     [Fact]
     public void Constructor_InitializesKey()
     {
         // Arrange & Act
-        var sut = new SecretItemViewModel("testKey", "testValue");
+        var sut = new SecretItemViewModel("testKey", "testValue", _mockClipboardService);
 
         // Assert
         sut.Key.ShouldBe("testKey");
@@ -20,7 +29,7 @@ public class SecretItemViewModelTests
     public void Constructor_InitializesActualValue()
     {
         // Arrange & Act
-        var sut = new SecretItemViewModel("testKey", "testValue");
+        var sut = new SecretItemViewModel("testKey", "testValue", _mockClipboardService);
 
         // Assert
         sut.ActualValue.ShouldBe("testValue");
@@ -30,7 +39,7 @@ public class SecretItemViewModelTests
     public void Constructor_InitializesIsValueVisibleToFalse()
     {
         // Arrange & Act
-        var sut = new SecretItemViewModel("testKey", "testValue");
+        var sut = new SecretItemViewModel("testKey", "testValue", _mockClipboardService);
 
         // Assert
         sut.IsValueVisible.ShouldBeFalse();
@@ -40,7 +49,7 @@ public class SecretItemViewModelTests
     public void DisplayValue_WhenNotVisible_ReturnsMaskedValue()
     {
         // Arrange
-        var sut = new SecretItemViewModel("testKey", "testValue");
+        var sut = new SecretItemViewModel("testKey", "testValue", _mockClipboardService);
 
         // Act & Assert
         sut.DisplayValue.ShouldBe("********");
@@ -50,7 +59,7 @@ public class SecretItemViewModelTests
     public void DisplayValue_WhenVisible_ReturnsActualValue()
     {
         // Arrange
-        var sut = new SecretItemViewModel("testKey", "testValue");
+        var sut = new SecretItemViewModel("testKey", "testValue", _mockClipboardService);
 
         // Act
         sut.IsValueVisible = true;
@@ -63,7 +72,7 @@ public class SecretItemViewModelTests
     public void ToggleVisibilityCommand_TogglesIsValueVisible()
     {
         // Arrange
-        var sut = new SecretItemViewModel("testKey", "testValue");
+        var sut = new SecretItemViewModel("testKey", "testValue", _mockClipboardService);
         sut.IsValueVisible.ShouldBeFalse();
 
         // Act
@@ -77,7 +86,7 @@ public class SecretItemViewModelTests
     public void ToggleVisibilityCommand_TogglesBackToHidden()
     {
         // Arrange
-        var sut = new SecretItemViewModel("testKey", "testValue");
+        var sut = new SecretItemViewModel("testKey", "testValue", _mockClipboardService);
         sut.IsValueVisible = true;
 
         // Act
@@ -88,16 +97,10 @@ public class SecretItemViewModelTests
     }
 
     [Fact]
-    public async Task CopyToClipboardCommand_RaisesOnCopyToClipboardEvent()
+    public async Task CopyToClipboardCommand_CallsClipboardService()
     {
         // Arrange
-        var sut = new SecretItemViewModel("testKey", "secretPassword");
-        string? copiedValue = null;
-        sut.OnCopyToClipboard += value =>
-        {
-            copiedValue = value;
-            return Task.CompletedTask;
-        };
+        var sut = new SecretItemViewModel("testKey", "secretPassword", _mockClipboardService);
 
         // Act
         sut.CopyToClipboardCommand.Execute(null);
@@ -105,14 +108,14 @@ public class SecretItemViewModelTests
         // Assert - need to wait for async handler
         // Give the async void handler time to complete
         await Task.Delay(100);
-        copiedValue.ShouldBe("secretPassword");
+        await _mockClipboardService.Received(1).SetTextAsync("secretPassword");
     }
 
     [Fact]
     public void IsValueVisible_SetterRaisesPropertyChangedForIsValueVisible()
     {
         // Arrange
-        var sut = new SecretItemViewModel("testKey", "testValue");
+        var sut = new SecretItemViewModel("testKey", "testValue", _mockClipboardService);
         var propertyChangedRaised = false;
         sut.PropertyChanged += (s, e) =>
         {
@@ -131,7 +134,7 @@ public class SecretItemViewModelTests
     public void IsValueVisible_SetterRaisesPropertyChangedForDisplayValue()
     {
         // Arrange
-        var sut = new SecretItemViewModel("testKey", "testValue");
+        var sut = new SecretItemViewModel("testKey", "testValue", _mockClipboardService);
         var propertyChangedRaised = false;
         sut.PropertyChanged += (s, e) =>
         {
@@ -150,7 +153,7 @@ public class SecretItemViewModelTests
     public void IsValueVisible_SetToSameValue_DoesNotRaisePropertyChanged()
     {
         // Arrange
-        var sut = new SecretItemViewModel("testKey", "testValue");
+        var sut = new SecretItemViewModel("testKey", "testValue", _mockClipboardService);
         sut.IsValueVisible = false; // Already false, but explicitly set
         var propertyChangedRaised = false;
         sut.PropertyChanged += (s, e) =>