Set additionalProperties when [JsonExtensionData] is present

[captainsafia]

Reacting to feedback in #56318 (comment) and discussion in #56707.

The additionalProperties keyword in a schema is used to indicate that a schema can match against properties explicitly defined in the schema.properties list and additional unspecified properties.

The JSON Schema specification that STJ's JsonSchemaExporter targets assumes that additionalProperties is true by default for all schemas, meaning that a schema for a type will always be able to capture properties that are not explicitly defined in the properties list (see here).

The OpenAPI specification (as of v3 that we target) assumes the opposite. additionalProperties is assumed false and must be explicitly enabled in order to indicate that a schema can capture properties not explicitly defined. Furthermore, OpenAPI doesn't support a value true for the schema to support catch-alls. It requires that you implement an empty object to indicate that additional property values can match any type.

These two problems come together like Voltron in our handling of [JsonExtensionData] attributes on property. JsonSchemaExporter assumes that additionalProperties is implicitly true and doesn't set it when it generates a schema. We have to patch this mismatch in our TransformSchemaNode implementation to comply with what OpenAPI expects.

Since [JsonExtensionData] can only ever be applied on Dictionary<string, object> and Dictionary<string, JsonElement>, we don't have to worry about specifying concrete schemas for the values of additional properties. The empty object will do.

[martincostello]

Suggested change

	// there is no need to encode a concrete schema for thm and the catch-all empty schema is sufficient.
	// there is no need to encode a concrete schema for them and the catch-all empty schema is sufficient.

[martincostello]

Is it worth having a test for the JsonElement case also mentioned in the description? Possibly also a class that derives from something with the base having the extension property?

[mikekistler]

The OpenAPI specification (as of v3 that we target) assumes the opposite. additionalProperties is assumed false ...

I don't think this is true. In the Schema Object section of the OpenAPI 3.0.3 spec it says:

additionalProperties - Value can be boolean or object. Inline or referenced schema MUST be of a Schema Object and not a standard JSON Schema. Consistent with JSON Schema, additionalProperties defaults to true.