fix(ci): use forks pool in vitest to avoid tinypool kill EPERM on Node v25

Root cause: tinypool worker teardown calls kill() which returns EPERM
on the act_runner host environment. Tests pass but the process crashes
during cleanup, causing CI to report failure. pool: 'forks' uses
child_process.fork() instead, avoiding the issue.

vitest.config.ts

@@ -5,6 +5,7 @@ export default defineConfig({
     globals: true,
     environment: 'node',
     passWithNoTests: true,
+    pool: 'forks',
     env: {
       DB_PROVIDER: 'memory',
       RATE_LIMIT_STORE_MODE: 'memory',