Fix RichTextValue serialization when nested in Dict or List

[erral]

This PR adds a global IJsonCompatible converter for IRichTextValue.

Previously, RichTextValue was only serializable when a context was available (via IContextawareJsonCompatible). However, when a RichTextValue is nested inside a Dict or List, the recursive json_compatible call is often made without a context, leading to a TypeError.

By adding a non-context-aware converter that defaults to using the .raw value, we provide a safe fallback that prevents these crashes.

[mister-roboto]

@erral thanks for creating this Pull Request and helping to improve Plone!

TL;DR: Finish pushing changes, pass all other checks, then paste a comment:

@jenkins-plone-org please run jobs

To ensure that these changes do not break other parts of Plone, the Plone test suite matrix needs to pass, but it takes 30-60 min. Other CI checks are usually much faster and the Plone Jenkins resources are limited, so when done pushing changes and all other checks pass either start all Jenkins PR jobs yourself, or simply add the comment above in this PR to start all the jobs automatically.

Happy hacking!