@jeanp413 thanks for this PR. While this approach makes sense I would like to minimize changes if possible.
I do not see a clear need for a new method notificationMessage, why not just reuse and enhance validationMessage? I think that will make it cleaner and leaner.

I tried that at first but it seemed like more changes at that time 😅
If I update the validationMessage signature to
validationMessage: (value: string) => { content: string, severity: Severity } | null;
I'll need to make some changes in tunnelView.ts

Another option could be string | { content: string, severity: Severity } | null to avoid touching tunnelView.ts

Thoughts?

I think making changes to the tunnelView is perfectly fine, if the changes are for the better.
@alexr00 fyi

Pushed some changes updating validationMessage return type

Works great, awesome work!
Merging in.