BUG: fix regression in Styler.export/use

[attack68]

• closes BUG: Styler export() and use() no longer work after Styler has been rendered. #39735

#39396 solved a bug that CSS was duplicated everytime a Styler rendered. The solution maintained the computed ctx object and cleared the _todo list of methods so it wasn't recalculated. This broke the export and use methods since they rely on passing methods from the _todo list.

This PR solves the above duplication problem by clearing the ctx object before each render, maintaining the _todo from one render to another, and therefore fixes the broken methods.

[jreback]

is there anything user visible that would have changed? can you make a test that exercises this? (i see you change the internal test which is good as well).

[attack68]

No the ctx obj is no documented, is complex and the user should never, and is unlikely, to be using it directly.

There was an existing test_export, it just didn't pick up the case where a Styler had been rendered so now this is modified.

[jreback]

cool thxs