perf(test): default to vmForks on Node 25; unstub envs

vitest.config.ts

@@ -18,6 +18,9 @@ export default defineConfig({
   test: {
     testTimeout: 120_000,
     hookTimeout: isWindows ? 180_000 : 120_000,
+    // Many suites rely on `vi.stubEnv(...)` and expect it to be scoped to the test.
+    // This is especially important under `pool=vmForks` where env leaks cross-file.
+    unstubEnvs: true,
     pool: "forks",
     maxWorkers: isCI ? ciWorkers : localWorkers,
     include: ["src/**/*.test.ts", "extensions/**/*.test.ts", "test/format-error.test.ts"],