ipynb - check if svg display data is encoded or not

We assumed svg mime type data was never encoded, but this is not the case when the SVG is produced by a pandoc convert `--to ipynb`. It happens in render context when embed is used. The svg data needs to be decoded before being written to the file at embedding time.

Author: cderv

src/core/jupyter/jupyter.ts

@@ -1930,9 +1930,11 @@ function mdImageOutput(
     ? (data as string[]).join("")
     : data as string;
 
-  // base64 decode if it's not svg
+  // base64 decode if it's not encoded svg
+  // when used in embed context, Pandoc will generate ipynb with base64 encoded svg data
+  // https://github.com/quarto-dev/quarto-cli/issues/9793
   const outputFile = join(options.assets.base_dir, imageFile);
-  if (mimeType !== kImageSvg) {
+  if (mimeType !== kImageSvg || !imageText.trimStart().startsWith("<svg")) {
     const imageData = base64decode(imageText);
 
     // if we are in retina mode, then derive width and height from the image