fix(streaming): settle usage after client disconnect

[drewstone]

What changed

• add an opt-in background continuation callback for OpenAI-compatible streams
• keep the sandbox iterator and payment settlement alive after a client closes its reader
• retain cancellation as the default when the callback is absent
• stop response writes after disconnect without stopping usage collection

Why

Agent-app keeps persisted chat turns alive after an API viewer disconnects. The gateway previously stopped consuming that turn, so it could miss the final usage receipt and retain payment ownership.

Proof

• Node 24: 447/447 tests pass
• Node 22.13: 447/447 tests pass
• focused middleware: 66/66 tests pass
• typecheck and build pass
• the new end-user cancel flow records input, output, reasoning, and provider cost after the reader closes
• the existing cancel-by-default flow still aborts the sandbox iterator