Fix dead code, improve error messages, and update CanParse in Mermaid parser

- Add MermaidDiagramKind.Unknown for recognized-but-unsupported Mermaid types
- Update TryDetectKind to return Unknown for known Mermaid keywords (sequenceDiagram,
  classDiagram, gantt, etc.) so they bypass the generic error and reach MermaidParser
- Fix dead null-parser guard in MermaidParser.Parse: now reachable for Unknown kind,
  emitting an informative message with the concrete type token
- Update MermaidParser.CanParse to check registered parsers (returns false for Unknown)
- Differentiate MermaidDocument.Parse error messages: whitespace vs. comment-only vs.
  unrecognized type
- Remove redundant `document is not null` guard; add [NotNullWhen(true)] to TryParse
- Use IndexOf/slice instead of Split for first-token extraction (avoids array allocation)

Co-authored-by: jongalloway <68539+jongalloway@users.noreply.github.com>

Author: Copilot

src/DiagramForge/Parsers/Mermaid/MermaidDiagramKind.cs

@@ -2,5 +2,6 @@ namespace DiagramForge.Parsers.Mermaid;
 
 internal enum MermaidDiagramKind
 {
+    Unknown,
     Flowchart,
 }

src/DiagramForge/Parsers/Mermaid/MermaidDocument.cs

@@ -17,7 +17,7 @@ private MermaidDocument(MermaidDiagramKind kind, string headerLine, string[] lin
 
     public string[] Lines { get; }
 
-    public static bool TryParse(string diagramText, out MermaidDocument? document)
+    public static bool TryParse(string diagramText, [System.Diagnostics.CodeAnalysis.NotNullWhen(true)] out MermaidDocument? document)
     {
         document = null;
         if (string.IsNullOrWhiteSpace(diagramText))
@@ -40,12 +40,50 @@ public static bool TryParse(string diagramText, out MermaidDocument? document)
         return true;
     }
 
+    // Known Mermaid diagram-type keywords (lowercased) that are recognized but not yet
+    // supported by any registered IMermaidDiagramParser. Detecting them as Unknown lets
+    // MermaidParser emit a specific "unsupported type" error instead of a generic one.
+    private static readonly HashSet<string> KnownUnsupportedMermaidKeywords = new(StringComparer.Ordinal)
+    {
+        "sequencediagram",
+        "classdiagram",
+        "statediagram",
+        "statediagram-v2",
+        "erdiagram",
+        "journey",
+        "gantt",
+        "pie",
+        "gitgraph",
+        "mindmap",
+        "timeline",
+        "quadrantchart",
+        "requirementdiagram",
+        "block-beta",
+        "packet-beta",
+        "architecture-beta",
+        "kanban",
+        "sankey-beta",
+        "xychart-beta",
+        "zenuml",
+    };
+
     public static MermaidDocument Parse(string diagramText)
     {
-        if (TryParse(diagramText, out var document) && document is not null)
+        if (TryParse(diagramText, out var document))
             return document;
 
-        throw new DiagramParseException("Diagram text cannot be null, empty, or an unsupported Mermaid diagram type.");
+        if (string.IsNullOrWhiteSpace(diagramText))
+            throw new DiagramParseException("Diagram text cannot be null or whitespace.");
+
+        var firstContentLine = diagramText
+            .Split('\n')
+            .Select(line => line.Trim())
+            .FirstOrDefault(line => !string.IsNullOrEmpty(line) && !line.StartsWith("%%", StringComparison.Ordinal));
+
+        if (firstContentLine is null)
+            throw new DiagramParseException("Diagram text cannot be empty or contain only comments.");
+
+        throw new DiagramParseException($"Unsupported Mermaid diagram type '{firstContentLine}'.");
     }
 
     private static bool TryDetectKind(string headerLine, out MermaidDiagramKind kind)
@@ -60,6 +98,14 @@ private static bool TryDetectKind(string headerLine, out MermaidDiagramKind kind
             return true;
         }
 
+        var spaceIndex = normalizedHeader.IndexOf(' ');
+        var keyword = spaceIndex >= 0 ? normalizedHeader[..spaceIndex] : normalizedHeader;
+        if (KnownUnsupportedMermaidKeywords.Contains(keyword))
+        {
+            kind = MermaidDiagramKind.Unknown;
+            return true;
+        }
+
         kind = default;
         return false;
     }

src/DiagramForge/Parsers/Mermaid/MermaidParser.cs

@@ -26,7 +26,10 @@ public sealed class MermaidParser : IDiagramParser
     /// <inheritdoc/>
     public bool CanParse(string diagramText)
     {
-        return MermaidDocument.TryParse(diagramText, out _);
+        if (!MermaidDocument.TryParse(diagramText, out var document))
+            return false;
+
+        return _diagramParsers.Any(p => p.CanParse(document.Kind));
     }
 
     /// <inheritdoc/>
@@ -36,8 +39,10 @@ public Diagram Parse(string diagramText)
         var parser = _diagramParsers.FirstOrDefault(candidate => candidate.CanParse(document.Kind));
         if (parser is null)
         {
+            var spaceIndex = document.HeaderLine.IndexOf(' ');
+            var diagramTypeToken = spaceIndex >= 0 ? document.HeaderLine[..spaceIndex] : document.HeaderLine;
             throw new DiagramParseException(
-                $"Mermaid diagram type '{document.Kind}' is not supported. " +
+                $"Unsupported Mermaid diagram type '{diagramTypeToken}'. " +
                 $"Supported Mermaid types: {string.Join(", ", SupportedDiagramTypes)}");
         }