Add cancel endpoint for background responses

[cdoern]

Summary

Add POST /v1/responses/{response_id}/cancel endpoint to cancel queued or in-progress background responses.

Motivation

The cancelled status already exists in the response schema but there is no API to trigger it. Users need a way to cancel background responses that are queued or still in progress.

Scope

• CancelResponseRequest model and protocol method
• Task tracking infrastructure to map response_id to asyncio.Task
• Background worker handles CancelledError gracefully
• Proper status transitions and terminal state protection (409 for completed/failed/incomplete responses)
• Idempotent cancellation support
• Integration tests covering all cancel scenarios

Tracking PR: #5051