refactor(deps-dev): bump vitest from 3.2.4 to 4.1.8

Major-version Vitest upgrade across the workspace. Refactors
server/vitest.config.ts to use top-level minWorkers/maxWorkers
(replacing nested poolOptions.forks.{maxForks,minForks}, which
Vitest 4 ignores at top level), updates scripts/run-vitest-stable.mjs
for the new CLI surface, and adds explicit vi.fn() generics required
by Vitest 4's stricter inference in two UI tests. Lockfile changes
intentionally excluded — refresh bot will regenerate.

packages/plugins/plugin-workspace-diff/package.json

@@ -63,7 +63,7 @@
     "react": "^19.0.0",
     "react-dom": "^19.0.0",
     "typescript": "^5.7.3",
-    "vitest": "^3.0.5"
+    "vitest": "^4.1.8"
   },
   "peerDependencies": {
     "react": ">=18",