bugfix: Add empty choices field to streaming usage chunks

[amd-youchen]

Motivation

The OpenAI API spec requires every streamed chunk to include a choices field. Our final usage-only chunks omitted it, causing strict clients (e.g. OpenAI Python SDK) to reject the response. This adds "choices": [] to all four usage chunk paths (chat/completion, single/fanout).

[github-actions]

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