Explicit additionalProperties: false should be emitted in OpenAPI 3.1 when AdditionalPropertiesAllowed = false #2646

Is your feature request related to a problem? Please describe.
Currently, when serializing OpenAPI 3.1 documents, setting AdditionalPropertiesAllowed = false on a schema does not emit "additionalProperties": false.

This causes a semantic issue in OpenAPI 3.1 because schemas follow JSON Schema 2020-12, where:

• Omitting additionalProperties defaults to an empty schema ({}), which allows any additional properties.
• Only explicitly setting "additionalProperties": false prevents extra properties, matching the user's intent.

Currently, OpenAPI.NET only emits additionalProperties: false for OpenAPI 3.0 documents, as demonstrated in #2631 and related tests. While this behavior is intentional per #2630, it does not preserve the explicit intent when targeting OpenAPI 3.1.

Official spec references:

• OpenAPI 3.1 Schema Object: inherits JSON Schema 2020-12 semantics
• JSON Schema 2020-12 `additionalProperties semantics: omission allows any additional properties

Describe the solution you'd like
When AdditionalPropertiesAllowed = false is explicitly set on a schema:

• OpenAPI.NET should emit "additionalProperties": false in OpenAPI 3.1 output.
• This ensures that the generated document accurately preserves user intent and aligns with JSON Schema 2020-12 semantics.
• Emitting this explicitly is safe and non-breaking for downstream consumers, as it only reinforces the intended constraint.

Describe alternatives you've considered

• Post-processing the OpenAPI 3.1 JSON/YAML to inject "additionalProperties": false manually for closed schemas.
  • This works but is a workaround and adds extra steps for consumers.
• Leaving the current behavior as-is.
  • This preservers the existing design choice, but loses semantic fidelity when targeting OpenAPI 3.1

Additional context

• Current behavior is explicitly tested in fix/additional properties to v3 #2631, which shows emission only for 3.0.
• fix: additional properties serialization should not emit a schema in v2 fix: additional properties serialization should not emit booleans in v3.1+ #2630 confirms that this behavior is intentional, but it does not account for JSON Schema 2020-12 semantics in OpenAPI 3.1.
• Explicitly emittion "additionalProperties": false would align the serializer with the official OpenAPI 3.1 spec and JSON Schema 2020-12 behavior, avoiding silent loss of user intent.