Add GetScrapePortals endpoint and related service functionality

- Introduced the GetScrapePortals method in the AI pipeline handler to list document scraping portals supported by the Opplens AI service.
- Updated the service layer to include GetScrapePortals, which retrieves the portals from the client and handles errors appropriately.
- Enhanced the routes to register the new endpoint for retrieving scrape portals.
- Added a new error type for invalid date ranges in the document scraper, improving validation and error handling.

This update expands the AI pipeline capabilities, allowing for better management of document scraping portals within the tender management system.

internal/document_scraper/errors.go

@@ -0,0 +1,8 @@
+package document_scraper
+
+import "errors"
+
+var (
+	// ErrInvalidDateRange is returned when created_at_from is after created_at_to.
+	ErrInvalidDateRange = errors.New("invalid date range: created_at_from must be before or equal to created_at_to")
+)