Bug: Folding problem with markdown source block

[Jack Kamm]

Hello,

I found that Org entries containing markdown source blocks don't get
properly folded on the main development branch, when markdown-mode is
also loaded.

To reproduce:

1. Download markdown-mode from MELPA or Github. [1]
2. Fix the paths in the attached init.el.
3. emacs -Q -l init.el test.org
4. Shift-tab to collapse the visibility

Output should look like this:

* Headline 1...

But instead I observe this:

* Headline 1... ```
... ```
...

If markdown-mode isn't loaded, then the problem goes away. I think the
problem might have to do with the fontification that markdown-mode
applies to the back-quoted code block.

Versions:
GNU Emacs 28.1 (build 2, x86_64-pc-linux-gnu, X toolkit, cairo version 1.17.6, 
Xaw3d scroll bars) of 2022-07-10
Org mode version 9.5.4 (release_9.5.4-717-g9cc60d @ 
/home/jack/dev/org-mode/lisp/)

[1] https://github.com/jrblevin/markdown-mode

(add-to-list 'load-path "~/.emacs.d/elpa/markdown-mode-20220708.6")
(require 'markdown-mode)

(add-to-list 'load-path "~/dev/org-mode/lisp")

* Headline 1
** Headline 1a

#+begin_src markdown
  Some source code:

  ```
  echo hello world
  ```

  A list:

  ,* Item 1
  ,* Item 2
#+end_src

** Headline 1b

Lorem ipsum.