fix: handle Linux root path in HTML export test

The test was checking that absolute paths (like Windows drive letters) are
not embedded in HTML output. However, on Linux, Path.GetPathRoot() returns
'/', which is commonly found in HTML content. Updated the test to skip this
check on non-Windows platforms.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: Copilot

src/tests/Graphify.Tests/Export/RelativePathHandlingTests.cs

@@ -333,7 +333,7 @@ public async Task HtmlExport_EmbeddedPaths_AreRelative()
 
         // Verify no absolute paths are embedded
         var drive = Path.GetPathRoot(_projectRoot);
-        if (!string.IsNullOrEmpty(drive))
+        if (!string.IsNullOrEmpty(drive) && drive != "/")
         {
             Assert.DoesNotContain(drive, html);
         }