feat: add line-based diff output for multiline string comparisons

TUnit.Assertions.Tests/Helpers/StringDifferenceTests.cs

@@ -77,4 +77,87 @@ at Assert.That(actual).IsEqualTo(expected)
         var exception = await Assert.That(sut).ThrowsException();
         await Assert.That(exception.Message.NormalizeLineEndings()).IsEqualTo(expectedMessage);
     }
+
+    [Test]
+    public async Task Multiline_String_Difference_Shows_Line_Diff()
+    {
+        var expected = "line 1\nline 2\nline 3";
+        var actual = "line 1\nchanged\nline 3";
+
+        var sut = async () => await Assert.That(actual).IsEqualTo(expected);
+
+        var exception = await Assert.That(sut).ThrowsException();
+        await Assert.That(exception.Message.NormalizeLineEndings())
+            .Contains("differences starting at line 2")
+            .And.Contains("- 2: line 2")
+            .And.Contains("+ 2: changed");
+    }
+
+    [Test]
+    public async Task Multiline_String_Shows_Context_Lines()
+    {
+        var expected = "line 1\nline 2\nline 3\nline 4\nline 5";
+        var actual = "line 1\nline 2\nchanged\nline 4\nline 5";
+
+        var sut = async () => await Assert.That(actual).IsEqualTo(expected);
+
+        var exception = await Assert.That(sut).ThrowsException();
+        // Should show context lines around the difference
+        await Assert.That(exception.Message.NormalizeLineEndings())
+            .Contains("1: line 1")
+            .And.Contains("2: line 2")
+            .And.Contains("- 3: line 3")
+            .And.Contains("+ 3: changed")
+            .And.Contains("4: line 4")
+            .And.Contains("5: line 5");
+    }
+
+    [Test]
+    public async Task Multiline_String_Handles_Added_Lines()
+    {
+        var expected = "line 1\nline 2";
+        var actual = "line 1\nline 2\nline 3";
+
+        var sut = async () => await Assert.That(actual).IsEqualTo(expected);
+
+        var exception = await Assert.That(sut).ThrowsException();
+        await Assert.That(exception.Message.NormalizeLineEndings())
+            .Contains("+ 3: line 3");
+    }
+
+    [Test]
+    public async Task Multiline_String_Handles_Removed_Lines()
+    {
+        var expected = "line 1\nline 2\nline 3";
+        var actual = "line 1\nline 2";
+
+        var sut = async () => await Assert.That(actual).IsEqualTo(expected);
+
+        var exception = await Assert.That(sut).ThrowsException();
+        await Assert.That(exception.Message.NormalizeLineEndings())
+            .Contains("- 3: line 3");
+    }
+
+    [Test]
+    public async Task Multiline_String_With_CRLF_Line_Endings()
+    {
+        var expected = "line 1\r\nline 2\r\nline 3";
+        var actual = "line 1\r\nchanged\r\nline 3";
+
+        var sut = async () => await Assert.That(actual).IsEqualTo(expected);
+
+        var exception = await Assert.That(sut).ThrowsException();
+        await Assert.That(exception.Message.NormalizeLineEndings())
+            .Contains("differences starting at line 2");
+    }
+
+    [Test]
+    public async Task Multiline_String_With_Case_Insensitive_Comparison()
+    {
+        var expected = "LINE 1\nline 2";
+        var actual = "line 1\nLINE 2";
+
+        // Should be equal with case-insensitive comparison
+        await Assert.That(actual).IsEqualTo(expected).IgnoringCase();
+    }
 }