BUG: fix regression in Styler.export/use
- [x] closes #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. 

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). 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. cool thxs