Room list: move MessagePreviewStore and previews into its own directory (#32710)

* refactor: move `MessagePreviewStore` and previews into its own directory

The `MessagePreviewStore` is used widly and not only by the room list.
Moving to its own folder to be able to remove old room list later with
less friction

* test: add more tests

apps/web/test/unit-tests/stores/message-preview/previews/PollStartEventPreview-test.ts

@@ -0,0 +1,37 @@
+/*
+Copyright 2024 New Vector Ltd.
+Copyright 2021 The Matrix.org Foundation C.I.C.
+
+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 { type MatrixClient } from "matrix-js-sdk/src/matrix";
+
+// Import directly from the file to avoid circular dependencies with MessagePreviewStore
+import { PollStartEventPreview } from "../../../../../src/stores/message-preview/previews/PollStartEventPreview";
+import { MatrixClientPeg } from "../../../../../src/MatrixClientPeg";
+import { makePollStartEvent } from "../../../../test-utils";
+
+jest.spyOn(MatrixClientPeg, "get").mockReturnValue({
+    getUserId: () => "@me:example.com",
+    getSafeUserId: () => "@me:example.com",
+} as unknown as MatrixClient);
+jest.spyOn(MatrixClientPeg, "safeGet").mockReturnValue({
+    getUserId: () => "@me:example.com",
+    getSafeUserId: () => "@me:example.com",
+} as unknown as MatrixClient);
+
+describe("PollStartEventPreview", () => {
+    it("shows the question for a poll I created", async () => {
+        const pollStartEvent = makePollStartEvent("My Question", "@me:example.com");
+        const preview = new PollStartEventPreview();
+        expect(preview.getTextFor(pollStartEvent)).toBe("My Question");
+    });
+
+    it("shows the sender and question for a poll created by someone else", async () => {
+        const pollStartEvent = makePollStartEvent("Your Question", "@yo:example.com");
+        const preview = new PollStartEventPreview();
+        expect(preview.getTextFor(pollStartEvent)).toBe("@yo:example.com: Your Question");
+    });
+});