test: add process-state isolation helper for reliability (#331)

tests/helpers-process-isolation.test.cjs

@@ -0,0 +1,41 @@
+const { test, describe } = require('node:test');
+const assert = require('node:assert/strict');
+const path = require('node:path');
+
+const { withIsolatedProcessState } = require('./helpers.cjs');
+
+describe('withIsolatedProcessState', () => {
+  test('restores env, cwd, and exitCode after callback', () => {
+    const originalCwd = process.cwd();
+    const originalExitCode = process.exitCode;
+    const originalMarker = process.env.GSD_TEST_ISOLATION_MARKER;
+
+    const tempCwd = path.dirname(originalCwd);
+
+    withIsolatedProcessState(() => {
+      process.env.GSD_TEST_ISOLATION_MARKER = 'changed';
+      process.exitCode = 73;
+      process.chdir(tempCwd);
+    });
+
+    assert.strictEqual(process.cwd(), originalCwd);
+    assert.strictEqual(process.exitCode, originalExitCode);
+    assert.strictEqual(process.env.GSD_TEST_ISOLATION_MARKER, originalMarker);
+  });
+
+  test('restores state even when callback throws', () => {
+    const originalCwd = process.cwd();
+    const originalPath = process.env.PATH;
+
+    assert.throws(() => {
+      withIsolatedProcessState(() => {
+        process.env.PATH = '';
+        process.chdir(path.dirname(originalCwd));
+        throw new Error('boom');
+      });
+    }, /boom/);
+
+    assert.strictEqual(process.cwd(), originalCwd);
+    assert.strictEqual(process.env.PATH, originalPath);
+  });
+});