Replace String() calls with .toString()/.toLocaleString() per ast-grep rule

- Add formatError() helper in CLI to safely handle unknown error types
- Add uniqueTestId() helper for generating unique test identifiers
- Replace String(id) with id.toString() for database ID conversions
- Replace String(n) with n.toLocaleString() for user-facing number formatting
- Fix TypeScript errors in test files (undefined checks, unused variables)
- Update lint commands to include ast-grep scanning

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

packages/testing/test-helpers/src/test-utils.ts

@@ -0,0 +1,15 @@
+/**
+ * Test utility functions
+ */
+
+/**
+ * Generates a unique test ID using timestamp and random string.
+ * Useful for creating unique emails, slugs, tokens, etc. in tests.
+ *
+ * @example
+ * const email = `user-${uniqueTestId()}@example.com`
+ * const slug = `org-${uniqueTestId()}`
+ */
+export function uniqueTestId(): string {
+  return `${Date.now().toString()}-${Math.random().toString(36).slice(2, 8)}`;
+}