Add a playwright test for backup reset / deleted (#28858)

* Add a playwright test for backup reset / deleted

A slightly tricky one to test but an important case that people can hit,
and one that otherwise wouldn't get hit a lot during normal usage, so I
think probably quite a useful test to have. Mostly though, I'm about
to change this to a toast, so I'd like a test to assert that it still works.

* Return directly

Co-authored-by: Florian Duros <florianduros@element.io>

* Fix tsdco

---------

Co-authored-by: Florian Duros <florianduros@element.io>

playwright/e2e/csAPI.ts

@@ -0,0 +1,48 @@
+/*
+Copyright 2024 New Vector Ltd.
+
+SPDX-License-Identifier: AGPL-3.0-only OR GPL-3.0-only
+Please see LICENSE files in the repository root for full details.
+*/
+
+import { APIRequestContext } from "playwright-core";
+import { KeyBackupInfo } from "matrix-js-sdk/src/crypto-api";
+
+import { HomeserverInstance } from "../plugins/homeserver";
+
+/**
+ * A small subset of the Client-Server API used to manipulate the state of the
+ * account on the homeserver independently of the client under test.
+ */
+export class TestClientServerAPI {
+    public constructor(
+        private request: APIRequestContext,
+        private homeserver: HomeserverInstance,
+        private accessToken: string,
+    ) {}
+
+    public async getCurrentBackupInfo(): Promise<KeyBackupInfo | null> {
+        const res = await this.request.get(`${this.homeserver.config.baseUrl}/_matrix/client/v3/room_keys/version`, {
+            headers: { Authorization: `Bearer ${this.accessToken}` },
+        });
+
+        return await res.json();
+    }
+
+    /**
+     * Calls the API directly to delete the given backup version
+     * @param version The version to delete
+     */
+    public async deleteBackupVersion(version: string): Promise<void> {
+        const res = await this.request.delete(
+            `${this.homeserver.config.baseUrl}/_matrix/client/v3/room_keys/version/${version}`,
+            {
+                headers: { Authorization: `Bearer ${this.accessToken}` },
+            },
+        );
+
+        if (!res.ok) {
+            throw new Error(`Failed to delete backup version: ${res.status}`);
+        }
+    }
+}