I would not be so sure with the 100%. France had a mutual defence treaty with Czechoslovakia in 1938. They not only chose not to honor it but U turned all the way and explicitly let Hitler occupy it.
Would you trust them in crunch time mere 90 years later?
That's the difference between having a supranational organization like the EU and NATO vs mere treaties. Also backstabbing was much more common back then, not so much now. The US is now uniquely upending the current regime by taking actions against allies and betraying their trust.
France will not risk Paris, but if Moscow launches a nuke on Tallinn and Estonia is in the French nuclear umbrella, then France will retaliate by launching its own nuke. That's what being under a nuclear umbrella means.
Of course, Russia won't waste its first strike on Tallinn. Their first strike will be aimed at French nuclear facilities if it comes to that. Again, that's what offering a nuclear umbrella entails.
Would you trust them in crunch time mere 90 years later?