Add additionalProperties for JsonExtensionData schemas

[mete0rfish]

Add additionalProperties for JsonExtensionData schemas

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Added support for JsonExtensionData in OpenAPI schema generation by setting additionalProperties for object schemas that accept extension data.

Description

OpenAPI schema generation was not reflecting JsonExtensionData properties, so DTOs that can deserialize undeclared JSON properties were still emitted as schemas without free-form additional properties.

This changes the schema generation logic to detect JsonTypeInfo object types with extension data and add an empty additionalProperties schema when one has not already been set. This preserves any existing additionalProperties value while making the generated schema match the runtime JSON behavior.

Fixes #66338

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