Fix endSnippet block getting removed for missing snippet (#397)

src/MarkdownSnippets/Processing/MarkdownProcessor.cs

@@ -281,7 +281,10 @@ void ProcessSnippetLine(Action<string> appendLine, List<MissingSnippet> missings
 
             MissingSnippet missing = new(key, line.LineNumber, line.Path);
             missings.Add(missing);
+            appendLine("```");
             appendLine($"** Could not find snippet '{key}' **");
+            appendLine("```");
+            appendLine("<!-- endSnippet -->");
         }
 
         bool TryGetSnippets(string key, string? relativePath, string? linePath, out IReadOnlyList<Snippet> snippetsForKey)

src/Tests/DirectoryMarkdownProcessor/InPlaceOverwriteWithFileSnippetMissing/file.md

@@ -0,0 +1,2 @@
+<!-- snippet: missing.txt -->
+<!-- endSnippet -->

src/Tests/DirectoryMarkdownProcessorTests.InPlaceOverwriteWithFileSnippetMissing.verified.md

@@ -0,0 +1,5 @@
+<!-- snippet: missing.txt -->
+```
+** Could not find snippet 'missing.txt' **
+```
+<!-- endSnippet -->

src/Tests/DirectoryMarkdownProcessorTests.cs

@@ -104,6 +104,25 @@ public Task InPlaceOverwriteUrlInclude()
         return Verifier.Verify(File.ReadAllTextAsync(result));
     }
 
+    [Fact]
+    public Task InPlaceOverwriteWithFileSnippetMissing()
+    {
+        var root = Path.GetFullPath("DirectoryMarkdownProcessor/InPlaceOverwriteWithFileSnippetMissing");
+        DirectoryMarkdownProcessor processor = new(
+            root,
+            convention: DocumentConvention.InPlaceOverwrite,
+            writeHeader: false,
+            directoryIncludes: _ => true,
+            markdownDirectoryIncludes: _ => true,
+            snippetDirectoryIncludes: _ => true,
+            treatMissingAsWarning: true);
+
+        processor.Run();
+
+        FileInfo fileInfo = new(Path.Combine(root, "file.md"));
+        return Verifier.VerifyFile(fileInfo);
+    }
+
     [Fact]
     public void ReadOnly()
     {