fix: inject stream_options.include_usage for Codex Chat streaming

Responses-to-Chat conversion did not set stream_options.include_usage,
so OpenAI-compatible upstreams (kimi/MiniMax, etc.) omitted the trailing
usage chunk in streaming mode. This caused token/cost/cache stats to be
recorded as zero for third-party streaming requests routed through the
Codex Chat path.

Inject include_usage when the request is streaming, merging into any
client-provided stream_options instead of overwriting it. Add tests for
the streaming, non-streaming, and merge cases.