fix: guard countMarkdownHeadings against non-string input

When returning from a subtask to the parent task, message.text can be a
non-string truthy value at runtime. The existing `if (!text)` guard only
catches falsy values, so `.replace()` throws TypeError on non-strings.

Add a `typeof text !== "string"` check to handle this defensively.

Fixes #11879

Author: roomote

webview-ui/src/utils/__tests__/markdown.spec.ts

@@ -8,6 +8,16 @@ describe("markdown heading helpers", () => {
 		expect(countMarkdownHeadings("")).toBe(0)
 	})
 
+	it("returns 0 for non-string values", () => {
+		// At runtime, message.text can be a non-string truthy value (e.g. when
+		// returning from a subtask). The function must handle this gracefully.
+		expect(countMarkdownHeadings(42 as unknown as string)).toBe(0)
+		expect(countMarkdownHeadings([] as unknown as string)).toBe(0)
+		expect(countMarkdownHeadings({} as unknown as string)).toBe(0)
+		expect(countMarkdownHeadings(true as unknown as string)).toBe(0)
+		expect(hasComplexMarkdown(123 as unknown as string)).toBe(false)
+	})
+
 	it("counts single and multiple headings", () => {
 		expect(countMarkdownHeadings("# One")).toBe(1)
 		expect(countMarkdownHeadings("# One\nContent")).toBe(1)

webview-ui/src/utils/markdown.ts

@@ -4,7 +4,7 @@
  * Code fences are stripped before matching to avoid false positives.
  */
 export function countMarkdownHeadings(text: string | undefined): number {
-	if (!text) return 0
+	if (!text || typeof text !== "string") return 0
 
 	// Remove fenced code blocks to avoid counting headings inside code
 	const withoutCodeBlocks = text.replace(/```[\s\S]*?```/g, "")