fix(whatsapp): convert Markdown bold/strikethrough to WhatsApp formatting (#14285)

* fix(whatsapp): convert Markdown bold/strikethrough to WhatsApp formatting

* refactor: Move `escapeRegExp` utility function to `utils.js`.

---------

Co-authored-by: Luna AI <luna@coredirection.ai>

src/markdown/whatsapp.test.ts

@@ -0,0 +1,62 @@
+import { describe, expect, it } from "vitest";
+import { markdownToWhatsApp } from "./whatsapp.js";
+
+describe("markdownToWhatsApp", () => {
+  it("converts **bold** to *bold*", () => {
+    expect(markdownToWhatsApp("**SOD Blast:**")).toBe("*SOD Blast:*");
+  });
+
+  it("converts __bold__ to *bold*", () => {
+    expect(markdownToWhatsApp("__important__")).toBe("*important*");
+  });
+
+  it("converts ~~strikethrough~~ to ~strikethrough~", () => {
+    expect(markdownToWhatsApp("~~deleted~~")).toBe("~deleted~");
+  });
+
+  it("leaves single *italic* unchanged (already WhatsApp bold)", () => {
+    expect(markdownToWhatsApp("*text*")).toBe("*text*");
+  });
+
+  it("leaves _italic_ unchanged (already WhatsApp italic)", () => {
+    expect(markdownToWhatsApp("_text_")).toBe("_text_");
+  });
+
+  it("preserves fenced code blocks", () => {
+    const input = "```\nconst x = **bold**;\n```";
+    expect(markdownToWhatsApp(input)).toBe(input);
+  });
+
+  it("preserves inline code", () => {
+    expect(markdownToWhatsApp("Use `**not bold**` here")).toBe("Use `**not bold**` here");
+  });
+
+  it("handles mixed formatting", () => {
+    expect(markdownToWhatsApp("**bold** and ~~strike~~ and _italic_")).toBe(
+      "*bold* and ~strike~ and _italic_",
+    );
+  });
+
+  it("handles multiple bold segments", () => {
+    expect(markdownToWhatsApp("**one** then **two**")).toBe("*one* then *two*");
+  });
+
+  it("returns empty string for empty input", () => {
+    expect(markdownToWhatsApp("")).toBe("");
+  });
+
+  it("returns plain text unchanged", () => {
+    expect(markdownToWhatsApp("no formatting here")).toBe("no formatting here");
+  });
+
+  it("handles bold inside a sentence", () => {
+    expect(markdownToWhatsApp("This is **very** important")).toBe("This is *very* important");
+  });
+
+  it("preserves code block with formatting inside", () => {
+    const input = "Before ```**bold** and ~~strike~~``` after **real bold**";
+    expect(markdownToWhatsApp(input)).toBe(
+      "Before ```**bold** and ~~strike~~``` after *real bold*",
+    );
+  });
+});