Responses API: implement resumable streaming and cancellation

[AlexCheema]

Context

The Responses API streaming events now include sequence_number fields per the OpenAI spec, but we don't yet implement the features that depend on them.

TODO

• Resumable streaming: Support the starting_after query parameter on GET /v1/responses/{id}?stream=true&starting_after={sequence_number} to resume a dropped stream. This requires persisting stream events (at least for the duration of the response).
• Response cancellation: Support POST /v1/responses/{id}/cancel to stop an in-progress response.

Notes

• sequence_number is already emitted on all streaming events (monotonically increasing per response)
• No event persistence exists yet — streams are currently fire-and-forget async generators

[Evanev7]

with cancellation merged, do you want to update the responses api to include this behaviour?