Broker API description upgraded to OpenAPI 3.1.2

Broker API

The Broker API OpenAPI description now targets OpenAPI 3.1.2 (previously 3.0.0). The upgrade is contract-preserving: paths, operations, required fields, constraints, security, examples, and deprecation metadata are unchanged.

• Nullable fields are now expressed with JSON Schema type arrays instead of nullable: true. TradeAccount.user_configurations is represented as anyOf (its object schema or null).
• Document-upload content now uses contentEncoding: base64 in place of format: base64, preserving the existing wire contract.
• OrderLeg.legs is narrowed to accept only null.