test: add unit tests for Durable Object and clean up test setup

- Add CanvasRoom broadcast/close/error tests with mock WebSockets
- Remove @cloudflare/vitest-pool-workers (incompatible with Vitest 4)
- Clean up vitest config (single config, no integration workspace)
- 71 tests across 7 test files, all passing

package.json

@@ -16,7 +16,6 @@
     "hono": "^4.7.6"
   },
   "devDependencies": {
-    "@cloudflare/vitest-pool-workers": "^0.14.7",
     "@sveltejs/vite-plugin-svelte": "^5.0.3",
     "svelte": "^5.28.2",
     "vite": "^6.3.2",