Fix CapacityLimiter._pending_borrowers leak on non-Cancelled exceptions

acquire_on_behalf_of only catches trio.Cancelled when cleaning up
_pending_borrowers.  If park() raises any other exception (e.g.
KeyboardInterrupt delivered via the abort mechanism), the task's
entry remains in _pending_borrowers permanently.  This is a slow
memory leak and can also cause _wake_waiters to encounter stale
entries.

Broaden the handler to BaseException and use pop with a default
to be safe against double-removal.

Author: bysiber

src/trio/_sync.py

@@ -377,8 +377,8 @@ async def acquire_on_behalf_of(self, borrower: Task | object) -> None:
             self._pending_borrowers[task] = borrower
             try:
                 await self._lot.park()
-            except trio.Cancelled:
-                self._pending_borrowers.pop(task)
+            except BaseException:
+                self._pending_borrowers.pop(task, None)
                 raise
         else:
             await trio.lowlevel.cancel_shielded_checkpoint()