This means that people are free to bomb Russian forces in Syria, and if Russia strikes back then it is an act of war yes? Equally, by your logic, we could bomb Russian forces in Ukraine right now and if Russia strikes back, only then is it an act of war. See what happens when you type without thinking.
Er... If French forces bomb Russian forces in Syria, then yes, it's an act of war by default, even before a Russian response. It's about the flag you are carrying at the time of the strike.
If French forces are pretending to be Syrian opposition and attack Russian forces, and the French pretend those are not French units, then it's not an act of war. That's a thin line they are walking there.
So if French forces come into an established warzone and conduct operations against the Russians, even without shooting first, then it means they are announcing participation in the war. If they are attacked, then any escalation depends on what the French plan to do after that.