fix: use Keys.FirstOrDefault() instead of PrimaryKey (null after JSON deserialization)

SiteMapping.PrimaryKey is not serialized in sitemap.json; after deserialization
it is null, causing ArgumentNullException in WordCountService constructor and
HomeController when calling Dictionary.ContainsKey(null).

Use mapping.Keys.FirstOrDefault() ?? mapping.PrimaryKey consistently, matching
the pattern already used in BookToolQueryService.

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

Author: BenjaminMichaelis

EssentialCSharp.Web/Controllers/HomeController.cs

@@ -34,16 +34,17 @@ public IActionResult Index()
             string headHtml = doc.DocumentNode.Element("html")?.Element("head")?.InnerHtml ?? string.Empty;
             string html = doc.DocumentNode.Element("html")?.Element("body")?.InnerHtml ?? string.Empty;
 
+            string pageKey = siteMapping.Keys.FirstOrDefault() ?? siteMapping.PrimaryKey ?? string.Empty;
             ViewBag.PageTitle = siteMapping.IndentLevel is 0 ? siteMapping.ChapterTitle + " " + siteMapping.RawHeading : siteMapping.RawHeading;
             ViewBag.NextPage = FlipPage(siteMapping!.ChapterNumber, siteMapping.PageNumber, true);
-            ViewBag.CurrentPageKey = siteMapping.PrimaryKey;
+            ViewBag.CurrentPageKey = pageKey;
             ViewBag.PreviousPage = FlipPage(siteMapping.ChapterNumber, siteMapping.PageNumber, false);
             ViewBag.HeadContents = headHtml;
             ViewBag.Contents = html;
-            ViewBag.PageWordCount = wordCountService.GetPageWordCount(siteMapping.PrimaryKey);
+            ViewBag.PageWordCount = wordCountService.GetPageWordCount(pageKey);
             ViewBag.ChapterWordCount = wordCountService.GetChapterWordCount(siteMapping.ChapterNumber);
-            ViewBag.WordsBeforePage = wordCountService.GetWordsBeforePage(siteMapping.PrimaryKey);
-            ViewBag.ChapterStartWords = wordCountService.GetChapterStartWords(siteMapping.PrimaryKey);
+            ViewBag.WordsBeforePage = wordCountService.GetWordsBeforePage(pageKey);
+            ViewBag.ChapterStartWords = wordCountService.GetChapterStartWords(pageKey);
             ViewBag.BookWordCount = wordCountService.GetBookWordCount();
             return View();
         }

EssentialCSharp.Web/Services/WordCountService.cs

@@ -43,8 +43,8 @@ public WordCountService(ISiteMappingService siteMappingService, IWebHostEnvironm
 
         foreach (SiteMapping mapping in orderedMappings)
         {
-            string pageKey = mapping.PrimaryKey;
-            if (_pageWordCounts.ContainsKey(pageKey))
+            string? pageKey = mapping.Keys.FirstOrDefault() ?? mapping.PrimaryKey;
+            if (pageKey is null || _pageWordCounts.ContainsKey(pageKey))
             {
                 // Multiple anchors on the same page; skip duplicates (already counted).
                 continue;