Align PathHelpers.SafePathCombine with TemplateDotNetTool secure implementation

src/DemaConsulting.BuildMark/PathHelpers.cs

@@ -34,6 +34,10 @@ internal static class PathHelpers
     /// <exception cref="ArgumentException">Thrown when relativePath contains invalid characters or path traversal sequences.</exception>
     internal static string SafePathCombine(string basePath, string relativePath)
     {
+        // Validate inputs
+        ArgumentNullException.ThrowIfNull(basePath);
+        ArgumentNullException.ThrowIfNull(relativePath);
+
         // Ensure the relative path doesn't contain path traversal sequences
         if (relativePath.Contains("..") || Path.IsPathRooted(relativePath))
         {

test/DemaConsulting.BuildMark.Tests/PathHelpersTests.cs

@@ -43,6 +43,30 @@ public void PathHelpers_SafePathCombine_ValidPaths_CombinesCorrectly()
         Assert.AreEqual(Path.Combine(basePath, relativePath), result);
     }
 
+    /// <summary>
+    ///     Test that SafePathCombine throws ArgumentNullException for null basePath.
+    /// </summary>
+    [TestMethod]
+    public void PathHelpers_SafePathCombine_NullBasePath_ThrowsArgumentNullException()
+    {
+        // Act & Assert
+        var exception = Assert.Throws<ArgumentNullException>(() =>
+            PathHelpers.SafePathCombine(null!, "subfolder/file.txt"));
+        Assert.AreEqual("basePath", exception.ParamName);
+    }
+
+    /// <summary>
+    ///     Test that SafePathCombine throws ArgumentNullException for null relativePath.
+    /// </summary>
+    [TestMethod]
+    public void PathHelpers_SafePathCombine_NullRelativePath_ThrowsArgumentNullException()
+    {
+        // Act & Assert
+        var exception = Assert.Throws<ArgumentNullException>(() =>
+            PathHelpers.SafePathCombine("/home/user/project", null!));
+        Assert.AreEqual("relativePath", exception.ParamName);
+    }
+
     /// <summary>
     ///     Test that SafePathCombine throws ArgumentException for path traversal with double dots.
     /// </summary>