Start Python SDK turn subscriptions at their attachment point (#44400)

## What changed

Make turn event delivery depend on when each consumer attaches:

- Handles returned by `thread.turn(...)` receive events from when the request is sent, including events received before the response arrives.
- Other handles and explicit low-level registrations receive only events from when they join. Remove replay of completed items, token usage, and terminal events; use `thread.read(include_turns=True)` for earlier history.
- Retain unread events for active subscribers and pending starts, release unused turn state, and close the returned subscription when an async start is cancelled.

Preserve the existing low-level subscription when repeated `turn_start(...)` calls return the same turn.

## Testing

Update coverage for late joins, request-time buffering, completion or transport failure before a start returns, async cancellation cleanup, and repeated low-level starts. Adjust external-message integration tests to allow handles to collect different items while receiving the same final response.

GitOrigin-RevId: 3b7f1268df1193dee88385006ae9510101086c24

sdk/python/docs/api-reference.md

@@ -233,6 +233,9 @@ passed to `run(...)` or `turn(...)` applies to that turn and subsequent turns.
 
 ## TurnHandle / AsyncTurnHandle
 
+A `thread.turn(...)` handle receives events from when the call sends its request.
+Other handles start when they join; use `thread.read(include_turns=True)` for earlier history.
+
 ### TurnHandle
 
 - `steer(input: str | Input) -> TurnSteerResponse`