List custom dictionary entries

When true, a successful JSON-object response additionally carries "usage": {"characters": <n>} — the billable character count this request submitted (0 for unbilled requests; the 2.5x adjustment for logographic languages is included, so CJK requests report more than their raw length). Repeat texts inside the billing deduplication window are still counted here even though they are not re-charged, so the number is an upper bound on the final charge. Responses whose body is not a JSON object (e.g. bare-array response formats) cannot carry the field. An existing usage field (such as account/status or completion token usage) is preserved unchanged.