Add LLM generation logic and tests

Author: theletterf

src/Elastic.Markdown/Myst/Renderers/LlmMarkdown/LlmBlockRenderers.cs

@@ -6,6 +6,7 @@
 using Elastic.Markdown.Myst.CodeBlocks;
 using Elastic.Markdown.Myst.Directives;
 using Elastic.Markdown.Myst.Directives.Admonition;
+using Elastic.Markdown.Myst.Directives.Diagram;
 using Elastic.Markdown.Myst.Directives.Image;
 using Elastic.Markdown.Myst.Directives.Include;
 using Markdig.Extensions.DefinitionLists;
@@ -372,6 +373,9 @@ protected override void Write(LlmMarkdownRenderer renderer, DirectiveBlock obj)
 			case IncludeBlock includeBlock:
 				WriteIncludeBlock(renderer, includeBlock);
 				return;
+			case DiagramBlock diagramBlock:
+				WriteDiagramBlock(renderer, diagramBlock);
+				return;
 		}
 
 		// Ensure single empty line before directive
@@ -417,6 +421,27 @@ private static void WriteImageBlock(LlmMarkdownRenderer renderer, ImageBlock ima
 		renderer.EnsureLine();
 	}
 
+	private static void WriteDiagramBlock(LlmMarkdownRenderer renderer, DiagramBlock diagramBlock)
+	{
+		renderer.EnsureBlockSpacing();
+
+		// Render diagram as structured comment with type information
+		renderer.WriteLine($"<diagram type=\"{diagramBlock.DiagramType}\">");
+
+		// Render the diagram content with indentation
+		if (!string.IsNullOrWhiteSpace(diagramBlock.Content))
+		{
+			var lines = diagramBlock.Content.Split(['\n', '\r'], StringSplitOptions.RemoveEmptyEntries);
+			foreach (var line in lines)
+			{
+				renderer.WriteLine(line);
+			}
+		}
+
+		renderer.WriteLine("</diagram>");
+		renderer.EnsureLine();
+	}
+
 	private void WriteIncludeBlock(LlmMarkdownRenderer renderer, IncludeBlock block)
 	{
 		if (!block.Found || block.IncludePath is null)

tests/authoring/LlmMarkdown/LlmMarkdownOutput.fs

@@ -488,3 +488,38 @@ Hello, this is a substitution: {{hello-world}}
 Hello, this is a substitution: Hello World!
 ```
 """
+
+type ``diagram directive`` () =
+    static let markdown = Setup.Document """
+::::{diagram} mermaid
+flowchart LR
+    A[Start] --> B{Decision}
+    B -->|Yes| C[Action 1]
+    B -->|No| D[Action 2]
+    C --> E[End]
+    D --> E
+::::
+
+::::{diagram} d2
+x -> y: hello world
+y -> z: nice to meet you
+::::
+"""
+
+    [<Fact>]
+    let ``renders diagram with type information`` () =
+        markdown |> convertsToNewLLM """
+<diagram type="mermaid">
+flowchart LR
+    A[Start] --> B{Decision}
+    B -->|Yes| C[Action 1]
+    B -->|No| D[Action 2]
+    C --> E[End]
+    D --> E
+</diagram>
+
+<diagram type="d2">
+x -> y: hello world
+y -> z: nice to meet you
+</diagram>
+"""