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/index.ts

@@ -16,3 +16,4 @@ export {
   truncateAllTables,
 } from "./test-db.js";
 export { withTestTransaction } from "./test-transaction.js";
+export { uniqueTestId } from "./test-utils.js";