MVVM - Introduce some helpers for snapshot management (#30398)

* Introduce snapshot class to track snapshot updates

This avoids the hassle of having to manually call emit.

* Better viewmodel ergonomics

- Rename `SubscriptionViewModel` to `BaseViewModel`. I feel this is
  appropriate since that class does more than just manage subscriptions.
- `getSnapshot` is no longer an abstract method. It's simply a method
  that returns the current snapshot state. This ensures that getSnapshot
result is cached by default which is required by `useSyncExternalStore`.
- `props` are a property of the base vm class so that actual VMs don't
  have to keep creating this property.

* Update `TextualEventViewModel`

* Fix test

* Rename `TextualEvent` to `TextualEventView`

* Fix snapshot object not being merged

* Rename directory to `EventTileView`

* Fix broken snapshot

* Add test for snapshot class

test/viewmodels/base/Snapshot-test.ts

@@ -0,0 +1,41 @@
+/*
+Copyright 2025 New Vector 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 { Snapshot } from "../../../src/viewmodels/base/Snapshot";
+
+interface TestSnapshot {
+    key1: string;
+    key2: number;
+    key3: boolean;
+}
+
+describe("Snapshot", () => {
+    it("should accept an initial value", () => {
+        const snapshot = new Snapshot<TestSnapshot>({ key1: "foo", key2: 5, key3: false }, jest.fn());
+        expect(snapshot.current).toStrictEqual({ key1: "foo", key2: 5, key3: false });
+    });
+
+    it("should call emit callback when state changes", () => {
+        const emit = jest.fn();
+        const snapshot = new Snapshot<TestSnapshot>({ key1: "foo", key2: 5, key3: false }, emit);
+        snapshot.merge({ key3: true });
+        expect(emit).toHaveBeenCalledTimes(1);
+    });
+
+    it("should swap out entire snapshot on set call", () => {
+        const snapshot = new Snapshot<TestSnapshot>({ key1: "foo", key2: 5, key3: false }, jest.fn());
+        const newValue = { key1: "bar", key2: 8, key3: true };
+        snapshot.set(newValue);
+        expect(snapshot.current).toStrictEqual(newValue);
+    });
+
+    it("should merge partial snapshot on merge call", () => {
+        const snapshot = new Snapshot<TestSnapshot>({ key1: "foo", key2: 5, key3: false }, jest.fn());
+        snapshot.merge({ key2: 10 });
+        expect(snapshot.current).toStrictEqual({ key1: "foo", key2: 10, key3: false });
+    });
+});

test/viewmodels/event-tiles/TextualEventViewModel-test.ts

@@ -8,10 +8,16 @@ Please see LICENSE files in the repository root for full details.
 import { MatrixEvent, MatrixEventEvent } from "matrix-js-sdk/src/matrix";
 
 import { TextualEventViewModel } from "../../../src/viewmodels/event-tiles/TextualEventViewModel";
+import { stubClient } from "../../test-utils";
+
+jest.mock("../../../src/TextForEvent.tsx", () => ({
+    textForEvent: jest.fn().mockReturnValue("Test Message"),
+}));
 
 describe("TextualEventViewModel", () => {
     it("should update when the sentinel updates", () => {
         const fakeEvent = new MatrixEvent({});
+        stubClient();
 
         const vm = new TextualEventViewModel({
             showHiddenEvents: false,