RichText: Combine nested `strong` tags to one

[hanneslsm]

What problem does this address?

Sometimes my client edit text and make it bold.
However, they somehow manage to create nested strong tags (I honestly don't know how but from my understanding they should't be able to when only using the editor). Because of this the variable font is rendered incorrectly.

Example

• First row: nested strong tags
  • 
• Second: manually removed to show you how the font should be rendered in Chrome (note the font-weight)
  • 

Which is displayed like this:

See: https://www.bittner-krull.de/produkte/rollout-planungstool/

What is your proposed solution?

The code should be cleaner by stripping strong tags / combining them to one if they are nested. And when one directly followed by another without any other characters in between.

[t-hamano]

This is very strange. I have tried to reproduce the problem but cannot. At least with the core inline formatting, this kind of nesting should not occur, and tags should be merged properly.

[hanneslsm]

I'll try to get more info on how my client edit texts and what exactly they do, so that we can reproduce this.

[kishanjasani]

@hanneslsm @t-hamano I was also not able to replicate: https://prnt.sc/3EmJTxDfvoio

Tested on Twenty Twenty Five theme.