test(fmt): add invalid duration style option test (denoland#6310)

Author: timreichen

fmt/duration_test.ts

@@ -1,6 +1,7 @@
 // Copyright 2018-2025 the Deno authors. MIT license.
-import { assertEquals, assertExists } from "@std/assert";
+import { assertEquals, assertExists, assertThrows } from "@std/assert";
 import { format } from "./duration.ts";
+import type { FormatOptions } from "./duration.ts";
 
 Deno.test({
   name: "format() handles duration since epoch",
@@ -90,3 +91,15 @@ Deno.test({
     assertEquals(format(16.342, { ignoreZero: true }), "16ms 342µs");
   },
 });
+
+Deno.test({
+  name: "format() throws on invalid style option",
+  fn() {
+    assertThrows(
+      () =>
+        format(16.342, { style: "invalid" as keyof FormatOptions["style"] }),
+      TypeError,
+      'style must be "narrow", "full", or "digital"!',
+    );
+  },
+});