feat: implement secure image upload pipeline and backend XSS guard (Stage 2 Task A)

tests/NexusReader.Application.Tests/Services/HtmlSanitizerServiceTests.cs

@@ -51,4 +51,20 @@ public class HtmlSanitizerServiceTests
         result.Should().NotContain("alert");
         result.Should().Contain("<img src=\"x\">");
     }
+
+    [Fact]
+    public void Sanitize_WithMarkdownCodeBlockContainingAngleBrackets_DoesNotStripAngleBrackets()
+    {
+        // Arrange
+        var service = new HtmlSanitizerService();
+        var input = "Here is some code:\n\n```csharp\nif (x < y && y > z) { Console.WriteLine(\"test\"); }\n```";
+
+        // Act
+        var result = service.Sanitize(input);
+
+        // Assert
+        result.Should().Contain("&lt;");
+        result.Should().Contain("&gt;");
+        result.Should().NotContain("<script>");
+    }
 }