Editing a link results in a new link being inserted, in addition to the original link

## Steps to reproduce

1. Move your cursor over an existing link
2. Click the Link toolbar button (or press Cmd+K) to bring up the link editor
3. Change the link to another link
4. Click Link

## Expected result

The URL of the existing link changes to the newly entered one.

## Actual result

A new link is inserted at the cursor point using the link URL as the link text as well as href. The original link still exists, and is in fact split into two separate links.

Here's a video demonstrating the problem.

https://github.com/user-attachments/assets/01e0fd95-d3dd-4f19-8730-29303f7fd271

Start by reproducing the issue through the Link toolbar button or Cmd+K, then trace the link editor's update handling. Done means changing an existing link updates its URL without inserting a new link or splitting the original.

Written by the indexing model from the issue text.