Fix mockLocale for vitest 4.x spyOn breaking change (#5303)

frontend/src/test-utils/mockLocale.ts

@@ -12,11 +12,31 @@ import { vi } from "vitest";
  * Defaults to `en-GB`
  */
 export const mockLocale = (defaultLocale = "en-GB"): void => {
-  const { DateTimeFormat } = Intl;
+  const OriginalDateTimeFormat = Intl.DateTimeFormat;
+
+  // Vitest 4.x requires function/class implementations for spyOn mocks when
+  // mocking constructors. For built-in constructors like Intl.DateTimeFormat
+  // that have internal slots, we use a function that returns a new instance.
+  // This is valid JavaScript - when a constructor returns an object, that
+  // object becomes the instance (instead of `this`).
+  function MockDateTimeFormat(
+    this: unknown,
+    locales?: Intl.LocalesArgument,
+    options?: Intl.DateTimeFormatOptions,
+  ): Intl.DateTimeFormat {
+    // Apply default locale when no locale is specified
+    return new OriginalDateTimeFormat(locales || defaultLocale, options);
+  }
+
+  // Inherit static methods from the original DateTimeFormat
+  Object.setPrototypeOf(MockDateTimeFormat, OriginalDateTimeFormat);
+  // Set up prototype chain so instanceof checks work correctly
+  Object.setPrototypeOf(
+    MockDateTimeFormat.prototype,
+    OriginalDateTimeFormat.prototype,
+  );
+
   vi.spyOn(Intl, "DateTimeFormat").mockImplementation(
-    (
-      locales?: Intl.LocalesArgument,
-      options?: Intl.DateTimeFormatOptions | undefined,
-    ) => new DateTimeFormat(locales || defaultLocale, options),
+    MockDateTimeFormat as typeof Intl.DateTimeFormat,
   );
 };