llama-server supports OpenAI-spec stream_options.include_usage since
b2b6b55a59 (Sep 2025); any server new enough for router mode sends the
final usage chunk when requested. With the flag hardcoded to false the
provider never requests it, so streamed responses record zero tokens
and session/context accounting stays empty.
Verified against llama-server b10094: with include_usage sent, the
final chunk carries usage (incl. prompt_tokens_details.cached_tokens);
without it, nothing.
