Have RTE manage markdown to html conversion in plain text mode

### Your use case

#### What would you like to do?
As of right now we are not really converting the markdown directly from RTE, but we are relying on the Rust SDK.
#### Why would you like to do it?
While this works very well most of the time, it does not allow supporto for things like mentions in plain text mode, even if the user suggestions are triggered
#### How would you like to achieve it?
We did some improvements in plain text mode and how markdown mode is parsed on the RTE, and we should now use such setting from RTE, and always send the formatted html body from the composer even in plain text mode, since the RTE will manage the markdown to html conversion.

### Have you considered any alternatives?

_No response_

### Additional context
This will make the markdown content have an html that is closer to how the RTE html works, it won't allow to have user suggestion be able to send mentions, but at least it won't trigger the user suggestion selection even if is not going to work properly. To allow that we need to have a way to recognise the suggestions in markdown mode.

Start by tracing the RTE and composer handling for plain text mode, especially where markdown is converted to HTML and where the Rust SDK is currently used. The change is complete when the composer sends the RTE-generated formatted HTML in plain text mode and the resulting content matches the stated RTE behavior, without attempting markdown mention recognition.

Written by the indexing model from the issue text.