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/tests/Api/api_coverage_manifest.php

@@ -25,6 +25,7 @@ return [
     'manual_operations' => [
         'GET /ping',
         'PUT /order',
+        'GET /orders/reference-suggestions',
     ],
     'happy_only_operations' => [
         'GET /ping',