Cancel Async Chat Stream

Cancel an active asynchronous chat stream.

This endpoint gracefully cancels an ongoing chat completion stream by:

1. Setting the cancellation token to stop the event generation loop
2. Cancelling the underlying asyncio task
3. Updating the stream status to 'cancelled'

Notes:

• Cancellation is irreversible - stream cannot be resumed
• Any active SSE connections will receive a final event and close
• Stream status will be updated to reflect cancellation
• Use DELETE endpoint to remove cancelled streams from storage