Undefined vs null fields from the OpenAPI specs

Hello team! Not sure where this issue belongs since I don't find a repository for the OpenAPI specs, but there are fields such as `message.usage.server_tool_use` that are defined as their type or `null` but the API actually sends answers where the field is omitted (in this case, if you don't use a server tool or explicitly ask to use it in the prompt). This leads to unexpected errors at runtime.