A provider's successful inference response: the raw text plus token usage when the API reported it.
The success value of LangExtract.Provider.infer/2. usage is nil
when the response carried no usage block (some proxies and local models
omit it); when present it carries :input_tokens and :output_tokens.
Summary
Types
@type usage() :: %{input_tokens: non_neg_integer(), output_tokens: non_neg_integer()}