Deeply nested block quotes throw inside MarkdownView's render #1004

Markdig's MarkdownPipelineBuilder.MaximumNestingDepth (128) is enforced during parsing: Markdown.Parse throws ArgumentException: Markdown elements in the input are too deeply nested for 128 or more nested block quotes. Probed on the #1000 branch: 127 nested > parse and render within the limit; 128, 129, 140 and 300 throw, in a plain MarkdownPipelineBuilder().Build() pipeline exactly as in the reader's.

MarkView's MarkdownViewer.RenderMarkdown calls Markdown.Parse from its Markdown property-changed handler, so the exception surfaces inside MarkdownView's render — for every flavor, chat included. A transcript line or comment body of 256 characters (> × 128) is enough. What the desktop app does with an exception escaping that handler is unverified; at best the view shows nothing for that item.

Independent of #982 / #1000, which budget only the nesting the HTML pass synthesizes; a tree deeper than the limit never reaches the pass because parsing throws first.

Candidate fixes, each its own design:

• MarkdownView.Render() catches the parse failure and shows the text as plain text (a TextBlock with the source), the same degrade the reader uses for HTML it cannot convert.
• Cap nesting before handing text to the viewer — cheaper, but it means a second parser.

Repro: new MarkdownView { Text = string.Concat(Enumerable.Repeat("> ", 128)) + "x" }.