fix: resolve remaining CodeQL issues from PR #876

- BookSearchTool: collapse else-if(!semanticAvailable)+else into single
  else with ternary to remove constant-condition CodeQL finding
- BookGuidelinesTool: use pattern match (is int / is GuidelineType) to
  extract nullable locals before lambda capture, removing nullable
  dereference warnings at lines 38 and 41

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

Author: BenjaminMichaelis

EssentialCSharp.Web/Tools/BookGuidelinesTool.cs

@@ -34,11 +34,11 @@ public string GetCSharpGuidelines(
 
         IEnumerable<GuidelineListing> filtered = _guidelinesService.Guidelines;
 
-        if (chapter.HasValue)
-            filtered = filtered.Where(g => g.ChapterNumber == chapter.Value);
+        if (chapter is int chapterValue)
+            filtered = filtered.Where(g => g.ChapterNumber == chapterValue);
 
-        if (typeFilter.HasValue)
-            filtered = filtered.Where(g => g.Type == typeFilter.Value);
+        if (typeFilter is GuidelineType typeFilterValue)
+            filtered = filtered.Where(g => g.Type == typeFilterValue);
 
         if (!string.IsNullOrWhiteSpace(keyword))
             filtered = filtered.Where(g =>

EssentialCSharp.Web/Tools/BookSearchTool.cs

@@ -312,13 +312,11 @@ public async Task<string> CheckTopicCoverage(
         {
             assessment = "**Mentioned** — referenced in book content but no dedicated section heading";
         }
-        else if (!semanticAvailable)
-        {
-            assessment = "**Not found in headings** — semantic search unavailable; topic may still be discussed in prose";
-        }
         else
         {
-            assessment = "**Not covered** — not found in section headings or semantic search";
+            assessment = semanticAvailable
+                ? "**Not covered** — not found in section headings or semantic search"
+                : "**Not found in headings** — semantic search unavailable; topic may still be discussed in prose";
         }
 
         sb.AppendLine(CultureInfo.InvariantCulture, $"**Assessment:** {assessment}");