Add test coverage for invoices, transactions, bookkeeping, and supplier
invoice API routes. Includes test helpers (createMockRequest,
parseJsonResponse, createMockRouteParams, createQueuedMockSupabase) and
fixture factories (makeInvoice, makeCustomer, makeSupplier,
makeSupplierInvoice, makeCompanySettings).
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>