fix: missing conversion to a SchemaObject

[snejugal]

The field json_schema: Schema on SchemaObject has #[serde(flatten)]. This is fine if this field is assigned the Schema::Object(..) variant, but there's also the other variant Schema::Bool(bool) which cannot be flattened. I'd wish to just fix the definition of SchemaObject to require a schemars::schema::SchemaObject and guarantee that no boolean scheme ever occurs there, but this field is public and thus changing its type is a breaking change. So for now I found a place where a boolean scheme may (and does in practice) occur and added an appropriate conversion.