fix(ui): catch marked.js parse errors to prevent Control UI crash (#36445)

- Prevent Control UI session render crashes when `marked.parse()` encounters pathological recursive markdown by safely falling back to escaped `<pre>` output.
- Tighten markdown fallback regression coverage and keep changelog attribution in sync for this crash-hardening path.

Co-authored-by: Bin Deng <dengbin@romangic.com>
Co-authored-by: Tak Hoffman <781889+Takhoffman@users.noreply.github.com>

ui/src/ui/markdown.test.ts

@@ -1,4 +1,5 @@
-import { describe, expect, it } from "vitest";
+import { marked } from "marked";
+import { describe, expect, it, vi } from "vitest";
 import { toSanitizedMarkdownHtml } from "./markdown.ts";
 
 describe("toSanitizedMarkdownHtml", () => {
@@ -82,4 +83,36 @@ describe("toSanitizedMarkdownHtml", () => {
     // Pipes from table delimiters must not appear as raw text
     expect(html).not.toContain("|------|");
   });
+
+  it("does not throw on deeply nested emphasis markers (#36213)", () => {
+    // Pathological patterns that can trigger catastrophic backtracking / recursion
+    const nested = "*".repeat(500) + "text" + "*".repeat(500);
+    expect(() => toSanitizedMarkdownHtml(nested)).not.toThrow();
+    const html = toSanitizedMarkdownHtml(nested);
+    expect(html).toContain("text");
+  });
+
+  it("does not throw on deeply nested brackets (#36213)", () => {
+    const nested = "[".repeat(200) + "link" + "]".repeat(200) + "(" + "x".repeat(200) + ")";
+    expect(() => toSanitizedMarkdownHtml(nested)).not.toThrow();
+    const html = toSanitizedMarkdownHtml(nested);
+    expect(html).toContain("link");
+  });
+
+  it("falls back to escaped plain text if marked.parse throws (#36213)", () => {
+    const parseSpy = vi.spyOn(marked, "parse").mockImplementation(() => {
+      throw new Error("forced parse failure");
+    });
+    const warnSpy = vi.spyOn(console, "warn").mockImplementation(() => {});
+    const input = `Fallback **probe** ${Date.now()}`;
+    try {
+      const html = toSanitizedMarkdownHtml(input);
+      expect(html).toContain('<pre class="code-block">');
+      expect(html).toContain("Fallback **probe**");
+      expect(warnSpy).toHaveBeenCalledOnce();
+    } finally {
+      parseSpy.mockRestore();
+      warnSpy.mockRestore();
+    }
+  });
 });

ui/src/ui/markdown.ts

@@ -110,11 +110,20 @@ export function toSanitizedMarkdownHtml(markdown: string): string {
     }
     return sanitized;
   }
-  const rendered = marked.parse(`${truncated.text}${suffix}`, {
-    renderer: htmlEscapeRenderer,
-    gfm: true,
-    breaks: true,
-  }) as string;
+  let rendered: string;
+  try {
+    rendered = marked.parse(`${truncated.text}${suffix}`, {
+      renderer: htmlEscapeRenderer,
+      gfm: true,
+      breaks: true,
+    }) as string;
+  } catch (err) {
+    // Fall back to escaped plain text when marked.parse() throws (e.g.
+    // infinite recursion on pathological markdown patterns — #36213).
+    console.warn("[markdown] marked.parse failed, falling back to plain text:", err);
+    const escaped = escapeHtml(`${truncated.text}${suffix}`);
+    rendered = `<pre class="code-block">${escaped}</pre>`;
+  }
   const sanitized = DOMPurify.sanitize(rendered, sanitizeOptions);
   if (input.length <= MARKDOWN_CACHE_MAX_CHARS) {
     setCachedMarkdown(input, sanitized);