RTE Link Picker: Fix media selection to allow items not permitted at root.

[engijlr]

Summary

• Fixed an issue where the RTE link picker only allowed selecting media items whose types were permitted at the media root level
• Users can now select media files inside folders regardless of root-level permissions
• Folders remain non-selectable

Changes:

1. input-media.context.ts: Added isUmbracoFolder() check to the pickableFilter to exclude folders from being selectable
2. link-picker-modal.element.ts: Removed the unnecessary #getMediaTypes() method and allowedContentTypeIds prop since folder exclusion is now handled at the filter level

Fixed: #18156

[AndyButland]

I've tested this one out and can confirm it works as expected. Previously I couldn't select "Image" types if I marked them as not allowed in the root, and now I can.

Please consider if you have any concerns or want second opinions on any of the front-end coding points @engijlr, and if so, we should wait for a FE-er review too. But from a functionality perspective, it looks good to me.

[engijlr]

Thanks for testing it @AndyButland. I think it would be nice if we could get an opinion from a front-ender, since I removed a function that, according to my tests and investigation, was redundant and the problematic one, but it might have been there for a reason that I couldn't see.

[nielslyngsoe]

This works and is right, but my review revealed another problem.
You can pick anything, but the RTE implementation only allows for inserting images+svg.

So I created this issue:
#21824