Refactor CommonMark instance creation
<!-- Please search existing work items before filing to avoid duplicates. -->
<!-- Also check open feature request work items to avoid duplicating existing proposals. -->
## Summary
Following #3586595 , we moved league/commonmark to be a real dependency for the AI module. In this we create CommonMarkConverter objects, all with different options.
```php
public function getCommonMarkConverter(): object {
return new CommonMarkConverter();
}
```
```php
$converter = new CommonMarkConverter(['html_input' => 'escape']);
```
```php
$converter = new CommonMarkConverter([
'html_input' => 'strip',
'allow_unsafe_links' => FALSE,
]);
```
It would be good to introduce some sort of service that would dependency inject the commonmarkconverter option so that the options that are injected are standardized correctly across the instance uses.
## Problem
Without dependency injection, anywhere CommonMarkConverter is used could use non-standard options, resulting in unexpected behavior.
<!-- /label ~"aiAgent" -->
<!-- /label ~"aiApiExplorer" -->
<!-- /label ~"aiAssistantsApi" -->
<!-- /label ~"aiAutomators" -->
<!-- /label ~"aiChatbot" -->
<!-- /label ~"aiCkeditor" -->
<!-- /label ~"aiContentSuggestions" -->
<!-- /label ~"aiEca" -->
<!-- /label ~"aiExternalModeration" -->
<!-- /label ~"aiLogging" -->
<!-- /label ~"aiObservability" -->
<!-- /label ~"aiSearch" -->
<!-- /label ~"aiTest" -->
<!-- /label ~"aiTranslate" -->
issue