Add unit tests for InvoicingPeriodDraftOverlay and reference suggestion logic, including fake DB integration and aggregation methods

- Implemented `InvoicingPeriodDraftOverlayTest` with coverage for blocking and permitting invoicing actions based on draft states, transactions, and metadata.
- Created `ReferenceSuggestionsApiTest` to validate ranked and filtered suggestions across bookings, orders, and vehicles with varied match relevance, context, and frequency.
- Added `order_reference_suggestions_service` class, including query methods, normalization utilities, and aggregation logic for reference suggestions.
- Enhanced query handling in `InvoicingPeriodDraftOverlayFakeDb` to validate SQL constraints and column cache resets in overlapping invoicing contexts.

services/nginx/app/objects/collected_order_invoices_o.php

@@ -508,6 +508,7 @@ class collected_order_invoices_o extends db
         }
         // Set the processor to E-conomic, if it's not already set to Stripe.
         $this->processor->set(ECONOMIC_PROCESSOR);
+        $this->error_message->nullify();
         // Object changed
         self::objectChanged();
         return $this;