allozaur/feat/chat form contenteditable (#26717)

* feat: Add contenteditable tokenizer for badge/code-chip chat input

* feat: Add source-space undo/redo history for the rich input

* feat: Split text glued to a closing code fence onto its own line

* feat: Add ChatFormContenteditable rich input renderer

* feat : wire the contenteditable into ChatForm with auto-switch gating

tools/ui/src/lib/components/app/content/MarkdownContent/MarkdownContent.svelte

@@ -34,7 +34,8 @@
 		preprocessLaTeX,
 		getImageErrorFallbackHtml,
 		copyCodeToClipboard,
-		copyToClipboard
+		copyToClipboard,
+		splitGluedClosingCodeFences
 	} from '$lib/utils';
 	import {
 		IMAGE_NOT_ERROR_BOUND_SELECTOR,
@@ -342,7 +343,11 @@
 	 * Incomplete code blocks are rendered using SyntaxHighlightedCode to maintain interactivity.
 	 * @param markdown - The raw markdown string to process
 	 */
-	async function processMarkdown(markdown: string) {
+	async function processMarkdown(rawMarkdown: string) {
+		// Text glued to a closing code fence is not a fence to the parser -
+		// the block would swallow it. Split it onto its own line first.
+		const markdown = splitGluedClosingCodeFences(rawMarkdown);
+
 		// Early exit if content unchanged (can happen with rapid coalescing)
 		if (markdown === previousContent) {
 			return;

tools/ui/src/lib/components/app/content/MarkdownContent/markdown-content.css

@@ -243,7 +243,6 @@ div.markdown-user-content :global(.table-wrapper) {
 /* Code blocks */
 
 .markdown-content :global(.code-block-wrapper) {
-	margin: 1.5rem 0;
 	border-radius: 0.75rem;
 	overflow: hidden;
 	border: 1px solid color-mix(in oklch, var(--border) 30%, transparent);
@@ -253,6 +252,14 @@ div.markdown-user-content :global(.table-wrapper) {
 	max-height: var(--max-message-height);
 }
 
+.markdown-content .markdown-block:not(:first-child) :global(.code-block-wrapper) {
+	margin-top: 1rem;
+}
+
+.markdown-content .markdown-block:not(:last-child) :global(.code-block-wrapper) {
+	margin-bottom: 1rem;
+}
+
 .markdown-content:global(.dark) :global(.code-block-wrapper) {
 	border-color: color-mix(in oklch, var(--border) 20%, transparent);
 }

tools/ui/src/lib/components/app/content/MarkdownContent/plugins/rehype/file-badge.ts

@@ -1,7 +1,7 @@
 /**
  * Rehype plugin that rewrites `file://` markdown anchors into the inline
- * @-mention chip, reusing the visual contract from
- * `$lib/constants/mention-badge`.
+ * mention chip, sharing the class string with the contenteditable
+ * tokenizer via `$lib/constants/mention-badge`.
  *
  * The chip is presentational: `file://` navigation is blocked from
  * http(s) pages, so the anchor becomes a plain `<span>` (no link role,