any open source tool to build a waiting list plus landing page while validating an idea ?

Skip that. Build it super fast. Make a gif. Put the idea in a GitHub gist. Spam all your SoMe networks with the gif and link to the gist. Add a visitor badge in the gist so you see how many check it out. I did this last week. Got 500 visits to a book draft I worked maybe half a day on.

Just find a no code landing page builder, they all have the wait list feature, or you can just made a tally form and embed to any page

Unicorn Platform by @johnrushx has a generous free tier that can help you validate an idea with minimum costs. It's quite aimed at that

Not an open-source project but it is free - in the past, I used ConvertKit for that. If you do not mind having their logo on your site, you will not need to pay anything. There is a limit of 1000 "subscribers" but possibly you can figure something out (export the list, save it locally, and start from 0).