perf(middleware): await background task cancellation on timeout

[ishaanxgupta]

Summary
Added an await asyncio.gather(*tasks_to_cancel, return_exceptions=True) call inside the except asyncio.TimeoutError: block of wait_for_background_tasks

Previously, when the wait_for_background_tasks method timed out, it iterated over unfinished tasks and called task.cancel(). However, calling cancel() only requests cancellation; it doesn't wait for the task's cleanup logic (e.g., except asyncio.CancelledError:) to actually complete. This created orphaned tasks running asynchronously in the background.
Failing to await cancelled tasks leads to "Task was destroyed but it is pending!" errors at shutdown. More importantly, it leaks memory and limits scalability because background cleanup continues executing unbounded.