python-client: client-initiated turn cancellation (cancel)

[brentrager]

Problem

The wire protocol and all five servers already honor the cancel frame / cancelled event, but the Python async client had no way to send it — the missing half of the just-merged TypeScript client (#459). There was no "Stop button" surface.

Solution

Mirror #459 in the Python client (python/src/smooth_operator/):

• SmoothAgentClient.cancel(request_id=..., session_id=None) — sends the {action: "cancel", requestId, sessionId?} frame (sync send, matching its confirm_tool_action / verify_otp siblings).
• MessageTurn.cancel() — ergonomic "stop THIS turn" convenience; cancels using the turn's own request_id + originating session_id.
• A terminal cancelled event settles the matching turn as a user-stop: it resolves (never raises), await turn yields the Cancelled event, the async for iterator ends cleanly, and turn.cancelled is True so a UI tells a stop apart from an error. Errors still raise ProtocolError.
• CancelRequest / Cancelled are now first-class in the ClientAction / ServerEvent discriminated unions, the ActionType / EventType enums, and the validator's schema maps.
• Idempotent: cancel with no active turn, or a cancelled with no matching in-flight turn, is a harmless no-op.

Reading user-stop vs error

turn = client.send_message(session_id=sid, message="write a novel")
turn.cancel()
final = await turn          # resolves — does NOT raise on a cancel
if turn.cancelled:          # final.type == "cancelled", final.status == 499
    ...                     # user stopped it
# an error still raises ProtocolError out of `await turn` / the async-for

Tests

5 new mock-transport tests in python/tests/test_client.py: turn/client cancel emit the correct frame, cancelled settles the matching turn as a user-stop (resolves, turn.cancelled == True, iterator ends cleanly), and idempotent no-ops (nothing running; late cancelled after completion). Full suite green (uv run pytest: 58 passed, 1 live-e2e skipped), ruff lint + format clean, compileall clean.

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Latest commit: ec3fbe5

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 2 packages

Name	Type
@smooai/smooth-operator	Minor
@smooai/smooth-operator-web-chat-example	Patch

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