test(01-05): add failing tests for TranslationSource singleton

- Instance singleton test
- EN string lookup test
- FR culture fallback test
- Missing key returns bracketed key
- PropertyChanged fires with empty string on culture switch
- Same culture does not fire PropertyChanged

SharepointToolbox.Tests/Localization/TranslationSourceTests.cs

@@ -1,7 +1,83 @@
+using SharepointToolbox.Localization;
+using System.ComponentModel;
+using System.Globalization;
+
 namespace SharepointToolbox.Tests.Localization;
 
-public class TranslationSourceTests
+[Trait("Category", "Unit")]
+public class TranslationSourceTests : IDisposable
 {
-    [Fact(Skip = "Wave 0 stub — implemented in plan 01-05")]
-    public void ChangeLanguage_Updates_Bound_Strings() { }
+    private readonly CultureInfo _originalCulture;
+
+    public TranslationSourceTests()
+    {
+        _originalCulture = TranslationSource.Instance.CurrentCulture;
+        // Reset to EN before each test
+        TranslationSource.Instance.CurrentCulture = new CultureInfo("en-US");
+    }
+
+    public void Dispose()
+    {
+        // Restore original culture after each test to prevent test pollution
+        TranslationSource.Instance.CurrentCulture = _originalCulture;
+    }
+
+    [Fact]
+    public void Instance_IsSameInstance_OnMultipleAccesses()
+    {
+        var instance1 = TranslationSource.Instance;
+        var instance2 = TranslationSource.Instance;
+        Assert.Same(instance1, instance2);
+    }
+
+    [Fact]
+    public void Indexer_ReturnsEnString_ForEnCulture()
+    {
+        TranslationSource.Instance.CurrentCulture = new CultureInfo("en-US");
+        var result = TranslationSource.Instance["app.title"];
+        Assert.Equal("SharePoint Toolbox", result);
+    }
+
+    [Fact]
+    public void Indexer_ReturnsFrOrFallback_AfterSwitchToFrFR()
+    {
+        TranslationSource.Instance.CurrentCulture = new CultureInfo("fr-FR");
+        var result = TranslationSource.Instance["app.title"];
+        // FR stub uses EN text — at minimum should not be null or empty
+        Assert.NotNull(result);
+        Assert.NotEmpty(result);
+        Assert.DoesNotContain("[", result); // Should not be missing-key placeholder
+    }
+
+    [Fact]
+    public void Indexer_ReturnsBracketedKey_ForMissingKey()
+    {
+        var result = TranslationSource.Instance["key.does.not.exist"];
+        Assert.Equal("[key.does.not.exist]", result);
+    }
+
+    [Fact]
+    public void ChangingCurrentCulture_FiresPropertyChanged_WithEmptyPropertyName()
+    {
+        string? capturedPropertyName = null;
+        TranslationSource.Instance.PropertyChanged += (sender, args) =>
+            capturedPropertyName = args.PropertyName;
+
+        TranslationSource.Instance.CurrentCulture = new CultureInfo("fr-FR");
+
+        Assert.Equal(string.Empty, capturedPropertyName);
+    }
+
+    [Fact]
+    public void SettingSameCulture_DoesNotFirePropertyChanged()
+    {
+        TranslationSource.Instance.CurrentCulture = new CultureInfo("en-US");
+        int fireCount = 0;
+        TranslationSource.Instance.PropertyChanged += (sender, args) => fireCount++;
+
+        // Set the exact same culture
+        TranslationSource.Instance.CurrentCulture = new CultureInfo("en-US");
+
+        Assert.Equal(0, fireCount);
+    }
 }