[BugFix] Make paper_bgcolor transparent in PyWry backend

[deeleeramone]

1. Why?:

   • Follow-up to this PR: [BugFix] Set Chart Style Before Output #6367

   • For display in PyWry, the dark/light toggle button on the mode bar requires a transparent background to work.

2. What? (1-3 sentences or a bullet point list):

   • The process in [BugFix] Set Chart Style Before Output #6367 sets the value so that it displays in a web browser or Jupyter environment. However, for display in PyWry, we need to make it transparent to avoid incompatibilities with the light/dark toggle button on the mode bar.

   • This updates the json_data immediately before output so that the Figure object itself is not modified on display and the Jupyter/browser view is preserved.

3. Impact (1-2 sentences or a bullet point list):

   • Should not have any operational impact.

4. Testing Done:

   • This can be tested by:
     • loading charts, then toggling dark/light on the mode bar.
     • Open the chart in a browser to verify the background and text are visible, res.chart.fig
     • Use the method for inverting the theme, res.charting.toggle_chart_style() and then display both ways again.