fix: add typeof guard in countMarkdownHeadings to prevent crash on non-string input

When message.text is a non-string truthy value (e.g. an array), calling
.replace() on it throws TypeError. This adds a typeof check so non-string
values are handled safely.

Fixes #11878

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

@@ -8,6 +8,15 @@ 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. array)
+		// which would cause .replace() to throw. The guard should handle this.
+		expect(countMarkdownHeadings(42 as unknown as string)).toBe(0)
+		expect(countMarkdownHeadings(["# heading"] as unknown as string)).toBe(0)
+		expect(countMarkdownHeadings({} as unknown as string)).toBe(0)
+		expect(countMarkdownHeadings(true as unknown as string)).toBe(0)
+	})
+
 	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, "")