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/scripts/update_sdk_artifacts.py

@@ -1294,8 +1294,8 @@ def _render_thread_block(turn_fields: list[PublicFieldSpec], *, is_async: bool =
         *_approval_mode_model_arg_lines(),
         *_model_arg_lines(turn_fields),
         "        )",
-        f"        turn = {await_prefix}{client}.turn_start(self.id, wire_input, params=params)",
-        f"        return {handle_type}({handle_owner}, self.id, turn.turn.id)",
+        f"        turn, subscription = {await_prefix}{client}._start_turn(self.id, wire_input, params=params, for_handle=True)",
+        f"        return {handle_type}({handle_owner}, self.id, turn.turn.id, _subscription=subscription)",
     ]
     return "\n".join(lines)