Make public procurement publication date optional in loan creation

Changes

This update makes the contract_publication_date field optional in the public procurement information when creating loans, reflecting that this data is publicly available elsewhere and does not need to be provided by partners. The contract_undertaking_date remains required as it is not publicly disclosed.

Updated endpoints

• POST /loans: The contract_publication_date property in the public procurement information schema is now nullable and no longer required. The contract_undertaking_date remains a required property.