BUG: Fix .rolling().mean() reassignment issue

This commit addresses a bug (issue #61841) in the pandas .rolling().mean() method, where reassigning the result of a rolling mean computation on the same column leads to unexpected NaN values. The root cause was incorrect alignment and slicing when using the step parameter inside the Window._apply() method. The fix ensures the result is properly computed and sliced only after the entire output is generated. Additionally, this update cleans up inconsistent import usage by moving Series and DataFrame imports to the top-level, which resolves pre-commit CI errors. After this fix, repeated assignments using .rolling().mean() behave as expected, and all pre-commit checks pass successfully.

Author: Abu Jabar Mubarak