Add title validation

Author: theletterf

src/Elastic.Documentation.Configuration/Builder/TableOfContentsConfiguration.cs

@@ -210,7 +210,6 @@ private IReadOnlyCollection<ITocItem> ReadChildren(YamlStreamReader reader, KeyV
 
 		if (crossLink is not null)
 		{
-			// No validation here - we'll validate cross-links separately
 			return [new CrossLinkReference(this, crossLink, title, hiddenFile, children ?? [])];
 		}
 

src/Elastic.Markdown/IO/Navigation/DocumentationGroup.cs

@@ -121,6 +121,12 @@ void AddToNavigationItems(INavigationItem item, ref int fileIndex)
 		{
 			if (tocItem is CrossLinkReference crossLink)
 			{
+				if (string.IsNullOrWhiteSpace(crossLink.Title))
+				{
+					context.EmitError(context.ConfigurationPath,
+						$"Cross-link entries must have a 'title' specified. Cross-link: {crossLink.CrossLinkUri}");
+					continue;
+				}
 				// Create a special navigation item for cross-repository links
 				var crossLinkItem = new CrossLinkNavigationItem(crossLink.CrossLinkUri, crossLink.Title, this, crossLink.Hidden);
 				AddToNavigationItems(crossLinkItem, ref fileIndex);