Add graceful cancel mode for streaming runs

[habema]

Resolves #1895

Solves the problem where canceling mid-turn discards partial work by adding an optional mode that waits for turn completion.

Implementation

API change (backward compatible):

def cancel(self, mode: Literal["immediate", "after_turn"] = "immediate")

Default is "immediate" (existing behavior). New "after_turn" mode sets a flag and lets the background task finish the current turn.

Core changes:

• result.py: Add _cancel_mode field and mode parameter to cancel()
• run.py: Add 3 checkpoint checks in streaming loop at top of turn loop, after handoff, and after tool execution

Checkpoints are placed after session saves, so when they trigger, the turn is already persisted.

How it works:

1. User calls cancel(mode="after_turn") - sets flag, doesn't clean up
2. Background task checks flag at next checkpoint
3. Marks complete and exits loop (cleanup happens in finally block as usual)

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[seratch]

LGTM @rm-openai let me know if you have any thoughts on this