Links for custom `OpenHandler` in markdown aren't supported in the AI Chat

[planger]

Steps to Reproduce:

1. Let an LLM output a markdown link for which an OpenHandler is registered, e.g. command:sample-command
2. Click the rendered link in the chat UI
3. Observe how the browser directly asks to Open xdg-open

Expected: The CommandOpenHandler should be triggered instead.

Additional Information

• Operating System: any
• Theia Version: master