Refactoring AIProvider and handling errors

[brichet]

This PR is to prepare adding new providers, see https://github.com/jupyterlite/jupyterlab-codestral/issues/13#issuecomment-2456729648 for context.

Changes

• The chat and completer providers are created every time a setting change, since some of them are not updating the config after init.
• The AIProvider is embedded in the chat handler
• If there is an error in the settings of the chat provider or while sending a message, the error message is displayed as a chat message. We may want a better way to display errors (notification, header in the chat...).

[brichet]

• We may want a better way to display errors (notification, header in the chat...).

AFAIK, the current implementation of the settings panel does not allow custom validator. The only way I can see to display a warning / error message in the settings panel is to add it in the DOM directly.