fix(tts): strip markdown before sending text to TTS engines (#13237)

Merged via /review-pr -> /prepare-pr -> /merge-pr.

Prepared head SHA: 163c68539f
Co-authored-by: danielwanwx <144515713+danielwanwx@users.noreply.github.com>
Co-authored-by: sebslight <19554889+sebslight@users.noreply.github.com>
Reviewed-by: @sebslight

src/tts/prepare-text.test.ts

@@ -0,0 +1,67 @@
+import { describe, expect, it } from "vitest";
+import { stripMarkdown } from "../line/markdown-to-line.js";
+
+/**
+ * Tests that stripMarkdown (used in the TTS pipeline via maybeApplyTtsToPayload)
+ * produces clean text suitable for speech synthesis.
+ *
+ * The TTS pipeline calls stripMarkdown() before sending text to TTS engines
+ * (OpenAI, ElevenLabs, Edge) so that formatting symbols are not read aloud
+ * (e.g. "hashtag hashtag hashtag" for ### headers).
+ */
+describe("TTS text preparation – stripMarkdown", () => {
+  it("strips markdown headers before TTS", () => {
+    expect(stripMarkdown("### System Design Basics")).toBe("System Design Basics");
+    expect(stripMarkdown("## Heading\nSome text")).toBe("Heading\nSome text");
+  });
+
+  it("strips bold and italic markers before TTS", () => {
+    expect(stripMarkdown("This is **important** and *useful*")).toBe(
+      "This is important and useful",
+    );
+  });
+
+  it("strips inline code markers before TTS", () => {
+    expect(stripMarkdown("Use `consistent hashing` for distribution")).toBe(
+      "Use consistent hashing for distribution",
+    );
+  });
+
+  it("handles a typical LLM reply with mixed markdown", () => {
+    const input = `## Heading with **bold** and *italic*
+
+> A blockquote with \`code\`
+
+Some ~~deleted~~ content.`;
+
+    const result = stripMarkdown(input);
+
+    expect(result).toBe(`Heading with bold and italic
+
+A blockquote with code
+
+Some deleted content.`);
+  });
+
+  it("handles markdown-heavy system design explanation", () => {
+    const input = `### B-tree vs LSM-tree
+
+**B-tree** uses _in-place updates_ while **LSM-tree** uses _append-only writes_.
+
+> Key insight: LSM-tree optimizes for write-heavy workloads.
+
+---
+
+Use \`B-tree\` for read-heavy, \`LSM-tree\` for write-heavy.`;
+
+    const result = stripMarkdown(input);
+
+    expect(result).not.toContain("#");
+    expect(result).not.toContain("**");
+    expect(result).not.toContain("`");
+    expect(result).not.toContain(">");
+    expect(result).not.toContain("---");
+    expect(result).toContain("B-tree vs LSM-tree");
+    expect(result).toContain("B-tree uses in-place updates");
+  });
+});