fix: strengthen type guard in countMarkdownHeadings to prevent TypeError on non-string input

Fixes #11605. The existing guard `if (!text)` only catches falsy values.
When a truthy non-string value (e.g. number or object from an API response)
is passed through OpenMarkdownPreviewButton, it passes the guard and
text.replace() throws "TypeError: n.replace is not a function".

Changed the guard to `typeof text !== "string"` so any non-string value
returns 0 early. Added test cases for non-string inputs.

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", () => {
+		// These simulate runtime scenarios where a non-string truthy value
+		// is passed due to unexpected API data (see issue #11605)
+		expect(countMarkdownHeadings(42 as unknown as string)).toBe(0)
+		expect(countMarkdownHeadings({ key: "value" } as unknown as string)).toBe(0)
+		expect(countMarkdownHeadings(["# heading"] as unknown as string)).toBe(0)
+		expect(countMarkdownHeadings(true as unknown as string)).toBe(0)
+		expect(countMarkdownHeadings(null 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 (typeof text !== "string" || !text) return 0
 
 	// Remove fenced code blocks to avoid counting headings inside code
 	const withoutCodeBlocks = text.replace(/```[\s\S]*?```/g, "")