Fix XDocument mutation and duplicate singleton relationships in WmlComparer

This commit addresses two issues:

1. XDocument Mutation (Issue OpenXmlDev#3):
   - WmlDocument.MainDocumentPart and PtMainDocumentPart.WordprocessingCommentsPart
     were removing nodes from cached XDocuments, causing subsequent GetXDocument()
     calls to return empty documents within the same package lifetime
   - Fixed by cloning the root element before extracting and removing child nodes
   - Location: WmlDocument.cs lines 67-81 and 20-37

2. Duplicate Singleton Relationships:
   - WmlComparer.MoveRelatedPartsToDestination was creating duplicate relationships
     for singleton parts (fontTable, styles, numbering, settings, webSettings, theme)
   - This caused validation errors: "can only have one instance of relationship
     that targets part fontTable"
   - Fixed by checking if a singleton relationship already exists and reusing it
     instead of creating a duplicate
   - Location: WmlComparer.cs MoveRelatedPartsToDestination method

Test Results:
- Fixed 2 WmlComparer tests (RC-0020, WCB-1740) that were failing with fontTable errors
- Improved from 22 failing tests to 20 failing tests (956 -> 958 passing)
- Remaining failures are unrelated to these issues

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: knilecrack

OpenXmlPowerTools/WmlComparer.cs

@@ -4771,6 +4771,30 @@ private static XElement MoveRelatedPartsToDestination(PackagePart partOfDeletedC
 
                 if (targetUri != null)
                 {
+                    // Check if this is a singleton relationship type that should be reused if it already exists
+                    var isSingletonRelationship =
+                        relationshipForDeletedPart.RelationshipType == "http://schemas.openxmlformats.org/officeDocument/2006/relationships/fontTable" ||
+                        relationshipForDeletedPart.RelationshipType == "http://schemas.openxmlformats.org/officeDocument/2006/relationships/styles" ||
+                        relationshipForDeletedPart.RelationshipType == "http://schemas.openxmlformats.org/officeDocument/2006/relationships/stylesWithEffects" ||
+                        relationshipForDeletedPart.RelationshipType == "http://schemas.openxmlformats.org/officeDocument/2006/relationships/numbering" ||
+                        relationshipForDeletedPart.RelationshipType == "http://schemas.openxmlformats.org/officeDocument/2006/relationships/settings" ||
+                        relationshipForDeletedPart.RelationshipType == "http://schemas.openxmlformats.org/officeDocument/2006/relationships/webSettings" ||
+                        relationshipForDeletedPart.RelationshipType == "http://schemas.openxmlformats.org/officeDocument/2006/relationships/theme";
+
+                    // For singleton relationships, check if one already exists and reuse it
+                    if (isSingletonRelationship)
+                    {
+                        var existingRelationship = partInNewDocument
+                            .GetRelationships()
+                            .FirstOrDefault(r => r.RelationshipType == relationshipForDeletedPart.RelationshipType);
+
+                        if (existingRelationship != null)
+                        {
+                            // Reuse the existing relationship ID
+                            att.Value = existingRelationship.Id;
+                            continue;
+                        }
+                    }
 
                     var relatedPackagePart = partOfDeletedContent.Package.GetPart(targetUri);
                     var uriSplit = relatedPackagePart.Uri.ToString().Split('/');

OpenXmlPowerTools/WmlDocument.cs

@@ -27,7 +27,8 @@ public PtWordprocessingCommentsPart WordprocessingCommentsPart
                 WordprocessingCommentsPart commentsPart = wDoc.MainDocumentPart.WordprocessingCommentsPart;
                 if (commentsPart == null)
                     return null;
-                XElement partElement = commentsPart.GetXDocument().Root;
+                // Clone the root element to avoid mutating the cached XDocument
+                XElement partElement = new XElement(commentsPart.GetXDocument().Root);
                 var childNodes = partElement.Nodes().ToList();
                 foreach (var item in childNodes)
                     item.Remove();
@@ -71,7 +72,8 @@ public PtMainDocumentPart MainDocumentPart
             using (MemoryStream ms = new MemoryStream(this.DocumentByteArray))
             using (WordprocessingDocument wDoc = WordprocessingDocument.Open(ms, false))
             {
-                XElement partElement = wDoc.MainDocumentPart.GetXDocument().Root;
+                // Clone the root element to avoid mutating the cached XDocument
+                XElement partElement = new XElement(wDoc.MainDocumentPart.GetXDocument().Root);
                 var childNodes = partElement.Nodes().ToList();
                 foreach (var item in childNodes)
                     item.Remove();