Bug: Backspace in Firefox can remove a whole LinkNode

Lexical version: v0.12.2

## Steps To Reproduce
https://github.com/facebook/lexical/assets/21961100/e7417126-2e6f-4db7-9326-800613939a0c

*only affects Firefox*
1. Type some text, e.g. 'test link here'
2. Make the link text a link
3. Navigate to the end of the link using the arrow left key or the mouse cursor
4. Hit backspace

## The current behavior
Removes the whole link node.

## The expected behavior
Removes only the last character of the text within the link.

Start by reproducing the steps in Firefox with Lexical v0.12.2, then trace the LinkNode and backspace handling involved when the cursor is at the link end. Done means backspace removes only the final character inside the link while preserving the LinkNode, without changing the behavior in other browsers.

Written by the indexing model from the issue text.