Rename callback on E2eSetup component (#31274)

* Rename callback on E2eSetup component

`BaseDialog.onFinished` is unused when `hasCancel=false`, so this callback is
only used when the user clicks cancel. For clarity, rename it.

* Test for cancellation behaviour

test/unit-tests/components/structures/auth/E2eSetup-test.tsx

@@ -0,0 +1,35 @@
+/*
+Copyright 2025 Element Creations Ltd.
+
+SPDX-License-Identifier: AGPL-3.0-only OR GPL-3.0-only OR LicenseRef-Element-Commercial
+Please see LICENSE files in the repository root for full details.
+*/
+
+import React from "react";
+import { render } from "jest-matrix-react";
+import { mocked } from "jest-mock";
+
+import E2eSetup from "../../../../../src/components/structures/auth/E2eSetup.tsx";
+import { InitialCryptoSetupStore } from "../../../../../src/stores/InitialCryptoSetupStore.ts";
+
+afterEach(() => jest.restoreAllMocks());
+
+describe("LeftPanel", () => {
+    it("should call `onCancelled` when the user clicks the cancel button", () => {
+        const mockInitialCryptoSetupStore = {
+            getStatus: jest.fn(),
+            on: jest.fn(),
+            off: jest.fn(),
+        };
+        jest.spyOn(InitialCryptoSetupStore, "sharedInstance").mockReturnValue(mockInitialCryptoSetupStore as any);
+
+        // We need the setup process to have failed, for the dialog to present a cancel button.
+        mocked(mockInitialCryptoSetupStore.getStatus).mockReturnValue("error");
+
+        const onCancelled = jest.fn();
+        const { getByRole } = render(<E2eSetup onCancelled={onCancelled} />);
+
+        getByRole("button", { name: "Cancel" }).click();
+        expect(onCancelled).toHaveBeenCalled();
+    });
+});