Data frame editing example doesn't work correctly

[wch]

From https://shiny.posit.co/py/api/express/express.render.data_frame.html (and the core version), the third example app doesn't work correctly. It is meant to save the changes, but it only saves the most recent change to the data, instead of all the cumulative changes.

The examples are at:

• https://github.com/posit-dev/py-shiny/blob/f732222ad981490ecaa698e7078e93c0ae6c6bd4/shiny/api-examples/data_frame_set_patches/app-express.py
• https://github.com/posit-dev/py-shiny/blob/f732222ad981490ecaa698e7078e93c0ae6c6bd4/shiny/api-examples/data_frame_set_patches/app-core.py

[wch]

@schloerke I changed the example in #1348 so that it works, but I feel like there could be a better way to do it. I'll merge and deploy now but we should revisit it when you are available.