test: update tests for multi-language translation support

- Update marketplace_product_fixtures to create translations
- Update test_marketplace_product.py for translation-based titles
- Update test_product.py for effective property tests
- Update test_order.py to use get_title() method
- Add comprehensive CSV processor tests for translations
- Update stats service tests for new flat response structure
- Fix product schema tests with required marketplace_product_id field
- Add helper function create_marketplace_product_with_translation()

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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

tests/unit/utils/test_data_validation.py

@@ -1,4 +1,4 @@
-# tests/test_data_validation.py
+# tests/unit/utils/test_data_validation.py
 import pytest
 
 from app.utils.data_processing import GTINProcessor, PriceProcessor