Closes #31257

Problem

Updating the fillstyle of a marker using Line2D.set_fillstyle() recreates the marker using MarkerStyle(self._marker.get_marker(), fs). This drops existing attributes stored in the original MarkerStyle, including the rotation transform. As a result, rotated markers lose their rotation after updating the fillstyle.

Cause

set_fillstyle() constructs a new MarkerStyle from the marker symbol. This discards internal attributes such as the transform that were present in the original MarkerStyle instance.

Solution

Use the MarkerStyle._with_attrs() factory method to construct the updated marker while preserving the attributes of the existing marker.
This ensures that properties such as transform, joinstyle, and capstyle remain unchanged when updating the fillstyle.

Changes

• Updated Line2D.set_fillstyle() to use MarkerStyle._with_attrs() instead of constructing a new MarkerStyle.
• Preserved marker attributes such as transform, joinstyle, capstyle and snap_threshold.
• Added a regression test to ensure that rotated markers retain their transform when fillstyle is updated.

Result

Rotated markers now maintain their rotation when set_fillstyle() is called, matching the expected behaviour described in the issue.