perf(markdown): skip impossible table parses (#128408)

Co-authored-by: Amp <amp@ampcode.com>

packages/markdown-core/src/tables.test.ts

@@ -1,11 +1,30 @@
 // Markdown Core tests cover tables behavior.
-import { describe, expect, it } from "vitest";
+import { beforeEach, describe, expect, it, vi } from "vitest";
 import { convertMarkdownTables } from "./tables.js";
 
+const markdownToIRWithMetaMock = vi.hoisted(() => vi.fn());
+
+vi.mock("./ir.js", async (importOriginal) => {
+  const actual = await importOriginal<typeof import("./ir.js")>();
+  markdownToIRWithMetaMock.mockImplementation(actual.markdownToIRWithMeta);
+  return { ...actual, markdownToIRWithMeta: markdownToIRWithMetaMock };
+});
+
 describe("convertMarkdownTables", () => {
+  beforeEach(() => {
+    markdownToIRWithMetaMock.mockClear();
+  });
+
   it("falls back to code rendering for block mode", () => {
     const rendered = convertMarkdownTables("| A | B |\n|---|---|\n| 1 | 2 |", "block");
 
     expect(rendered).toBe("```\n| A | B |\n| --- | --- |\n| 1 | 2 |\n```");
   });
+
+  it("does not parse ordinary text that cannot contain a table", () => {
+    const text = "Ordinary iMessage reply with **bold** and _emphasis_.";
+
+    expect(convertMarkdownTables(text, "code")).toBe(text);
+    expect(markdownToIRWithMetaMock).not.toHaveBeenCalled();
+  });
 });

packages/markdown-core/src/tables.ts

@@ -13,7 +13,7 @@ const MARKDOWN_STYLE_MARKERS = {
 
 /** Converts markdown tables into the configured plaintext/code rendering mode. */
 export function convertMarkdownTables(markdown: string, mode: MarkdownTableMode): string {
-  if (!markdown || mode === "off") {
+  if (!markdown || mode === "off" || !markdown.includes("|")) {
     return markdown;
   }
   const effectiveMode = mode === "block" ? "code" : mode;