fix(client): suppress stale task notifications between turns in receive_response()

[qozle]

Fixes #788.

What happened

When a background task (spawned via run_in_background=True) completed between turns, its TaskNotificationMessage sat in the shared message buffer. The next receive_response() call would yield it as the first message — before any response to the new query. In some cases this caused the model to respond to the stale task context instead of the user's new prompt.

Root cause

ClaudeSDKClient uses a single persistent anyio.MemoryObjectReceiveStream for the lifetime of the connection. receive_response() stops iterating after a ResultMessage, but the buffer keeps accumulating messages. A background task that completes between Turn N's result and Turn N+1's first response injects its notification into that buffer with no turn marker.

Fix

Two additions to ClaudeSDKClient:

• _current_turn: int — incremented on every ResultMessage seen by receive_response()
• _task_turn_map: dict[str, int] — maps task_id to the turn it was started in

In receive_response(), task lifecycle events (TaskStartedMessage, TaskNotificationMessage) that arrive before the first non-task message of the current turn are deferred. Once the first substantive message arrives:

• Deferred TaskNotificationMessage entries for tasks started in a previous turn are discarded (stale cross-turn noise).
• Everything else is flushed in order, then the iterator continues normally.

Task completions that arrive mid-turn (after the first AssistantMessage) are still yielded. The map entry is removed after the notification is processed to avoid unbounded growth on long-lived clients.

receive_messages() is not changed — callers that need every event unfiltered should use that method.

Tests

Four new cases in TestReceiveResponseTaskNotificationHygiene:

1. Stale notification buffered before Turn 2 starts → suppressed
2. Notification arriving after the first AssistantMessage → yielded
3. Turn counter increments; map entry cleaned up after notification
4. Notification for unknown task_id (no TaskStartedMessage seen) → yielded (safe default)

All 440 existing tests pass.