The previous formula had errors and overall code is now simplified.
Added a new section for calculating average of values in a array with missing values.
I can confirm that this issue is now resolved with the latest Drupal 10 and CK editor 5. The following is a step by step guide to achieve this.
Step 1: Go to Configuration > Content Authoring > Text Formats
Step 2: Create a new text format (I would advocate creating a new format which will give you finer control over what the editor can see in the toolbar. I have created a new format called Webform Editing.
Step 3: Assign the roles which can use this text editor. Be careful here so that everyone does not get access to this text format.
Step 4: Choose the CKEditor 5 from the text editor dropdown
Step 5: Drag and drop the buttons that you want. The media button looks like a Image with an audio symbol on it. Not very intuitive and to avoid confusions I have not the image button which actually looks quite like the media button.
Step 6: Click the Embed Media checkbox in the Enabled Filters section.
Step 7: In the filter settings please selec the type of media entities that can be embedded and the view modes for the said entities.
Once done you have created a text format that can be used. Now we need to ensure that webform users can use this text format. In order to do so please go to Structure > Webforms > Configuration > Elements.
Scroll down till you reach the HTML editor settings and in the element text format choose the Webform Editing Text format. You can also choose the mail text format. Dont forget to scroll down and save.
Now all description text elements in your HTML editor will have the media button which will enable you to use the Media browser to select and embed media that you wish.