Collab: Active RichText formats can reset unexpectedly

Active RichText formats (like bold, italics, inline code) can be deactivated when a peer edit comes in, or if there is a peer caret in the same block.

## Steps to reproduce

1. In one client, hit the Bold button and start typing in bold.
2. On the other client, type something in a different block.
3. Go back to the other client, and it's not in Bold mode anymore.

https://user-images.githubusercontent.com/555336/133293803-08ed36f8-e37c-4d4f-956d-a9336c41c418.mp4

Start by reproducing the two-client scenario described in the issue, including a peer caret in the same block, then trace where active RichText format state changes after a remote edit. Done means bold, italics, inline code, and other active formats remain enabled while collaborating.

Written by the indexing model from the issue text.