Chore: Upgrade Vitest to v4 (#7797)

## Summary

https://vitest.dev/guide/migration.html#vitest-4

## Changes

- **What**: Update Vitest and some associated dependencies
- **What**: Fix issue with our existing mocks and mock types

## Review Focus

Double check the test updates. I tried to keep the changes minimal.

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-7797-Chore-Upgrade-Vitest-to-v4-2d96d73d3650810cbe3ac42d7bd6585a)
by [Unito](https://www.unito.io)

---------

Co-authored-by: GitHub Action <action@github.com>

tests-ui/tests/api.featureFlags.test.ts

@@ -24,7 +24,9 @@ describe('API Feature Flags', () => {
     }
 
     // Mock WebSocket constructor
-    global.WebSocket = vi.fn().mockImplementation(() => mockWebSocket) as any
+    vi.stubGlobal('WebSocket', function (this: WebSocket) {
+      Object.assign(this, mockWebSocket)
+    })
 
     // Reset API state
     api.serverFeatureFlags = {}