Streaming runs can't be canceled gracefully

[habema]

Problem

When canceling a streaming run with result.cancel(), it stops immediately and discards the current turn. This means:

• The incomplete turn isn't saved to the session
• Usage tracking is cut off mid-turn
• Can't resume the session cleanly

This is painful if the user interrupts a long task - all progress from current turn is lost, and its usage cant be tracked.

Current behavior

result = Runner.run_streamed(agent, "Task", session=session)
async for event in result.stream_events():
    if timeout:
        result.cancel()  # Stops immediately, turn is discarded

Would be nice to have a way to let the current turn complete before stopping.