fix: Guard ReentrantLock against None task context (#841)

Add a runtime guard in `ReentrantLock` to raise `RuntimeError` when
`asyncio.current_task()` returns `None`, preventing the lock from being
silently bypassed due to `None is None` evaluating to `True`.

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: vdusek

src/apify/_utils.py

@@ -141,6 +141,8 @@ def __init__(self) -> None:
     async def __call__(self) -> AsyncIterator[None]:
         """Acquire the lock if it's not already owned by the current task, otherwise proceed without acquiring."""
         me = asyncio.current_task()
+        if me is None:
+            raise RuntimeError('ReentrantLock must be used within an asyncio.Task')
         if self._owner is me:
             yield
             return