AdditionalPropertiesAllowed is not serialized into additionalProperties even when explicitly set to false

[shazamViz]

Describe the bug

When generating an OpenAPI document using Microsoft.OpenApi, setting
OpenApiSchema.AdditionalPropertiesAllowed = false does not result in
additionalProperties: false being emitted in the generated OpenAPI document.

As a result, it is currently impossible to generate a "closed" object schema
(using additionalProperties: false) through the public API, even though
AdditionalPropertiesAllowed exists on OpenApiSchema.

This leads to incorrect OpenAPI output and breaks downstream tools such as
NSwag, OpenAPI Generator, Kiota, etc.

Expected behavior

When AdditionalPropertiesAllowed is explicitly set to false, the generated
OpenAPI should contain:

additionalProperties: false