Bug 2012458 - Add policy for configuring sidebar AI providers.

[mkaply]

This also has a change to allow external icons to work in the sidebar.

I focused on adding/removing providers and removing prompts.

If we need to add prompts, it would be straightforward, but that wasn't part of the original requirements.

[1rneh]

Thanks for the patch! This is my first round of review. I wasn’t previously familiar with the GenAI system module or the sidebar integration, so these comments are based on my current understanding.

I’ve included some questions and a few concerns regarding how prompts and providers are currently being cleared.

[mkaply]

I have a couple more things to fix, but this is basically a rewrite that makes things much simpler and will make things much easier to undo.