Use fetch() in a way that works for file URLs (#3071)

fetch returns a response code of 0 when it successfully loads a `file://` resource.

This means we can't just rely on `response.ok`.

Required for https://github.com/element-hq/element-call/issues/2994

src/utils/fetch.test.ts

@@ -0,0 +1,30 @@
+/*
+Copyright 2025 New Vector Ltd.
+
+SPDX-License-Identifier: AGPL-3.0-only OR LicenseRef-Element-Commercial
+Please see LICENSE in the repository root for full details.
+*/
+
+import { expect, describe, it } from "vitest";
+
+import { isFailure } from "./fetch";
+
+describe("isFailure", () => {
+  it("returns false for a successful response", () => {
+    expect(isFailure({ ok: true, url: "https://foo.com" } as Response)).toBe(
+      false,
+    );
+  });
+
+  it("returns true for a failed response", () => {
+    expect(isFailure({ ok: false, url: "https://foo.com" } as Response)).toBe(
+      true,
+    );
+  });
+
+  it("returns false for a file:// URL with status 0", () => {
+    expect(
+      isFailure({ ok: false, url: "file://foo", status: 0 } as Response),
+    ).toBe(false);
+  });
+});