ipynb - check if svg display data is encoded or not

[cderv]

fixes #9793

We assumed svg mime type data was never encoded.

However, this is not the case when the Notebook file is produced by a pandoc convert --to ipynb. In this case, Pandoc will base64 encode the svg file. The svg data needs to be decoded before being written to the file at embedding time.Welcome to the quarto GitHub repo!

For Quarto, this will mostly happens in render context when embed is used as we generate this type of ipynb document internally.
It could also happens if a Python code in cell would also output a display_data with svg mime type and encoded data. It does not seem to be the common usage.

This is quite a narrow fix I believe, and as mentioned in #9793 , this is doing the same kind of check that we do in another function

quarto-cli/src/core/jupyter/jupyter.ts

Lines 991 to 997 in 122b0f3

	// base 64 decode if its not svg
	if (!imageText.trimStart().startsWith("<svg")) {
	const imageData = base64decode(imageText);
	Deno.writeFileSync(outputFile, imageData);
	} else {
	Deno.writeTextFileSync(outputFile, imageText);
	}

cc @cwickham if you want to try on your side, though I added your example as a test