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.

scripts/run-vitest-stable.mjs

@@ -60,7 +60,6 @@ const generalGroupNames = [generalServerGroupName, generalWorkspacesAGroupName,
 const serializedServerVitestArgs = [
   "--no-file-parallelism",
   "--maxWorkers=1",
-  "--minWorkers=1",
 ];
 
 function walk(dir) {
@@ -322,7 +321,7 @@ function runSerializedSuites(routeTests, shardIndex, shardCount) {
         "@paperclipai/server",
         routeTest.repoPath,
         "--pool=forks",
-        "--poolOptions.forks.isolate=true",
+        "--isolate",
       ],
       routeTest.repoPath,
     );