Warranty on lock damaged during attempted theft
Warranties exist to protect you, the consumer, against defects in materials or workmanship, not against deliberate damage. The purpose of a bike lock is to help avoid your bike being stolen, so if it has prevented a theft of the bicycle then it has done its job well. 

Perhaps you can ask Decathlon and report back to us by answering your own question for future readers, that would be most useful.

You would have to explore if the damage is covered by any of your insurances, check local laws on any time limits to report the attempted theft by (to police etc).

Sometimes lock companies will offer private schemes to replace stolen goods, like this number one Google hit, but you would probably know about such a scheme if it was advertised on your product and you'd likely need to have registered the purchase. 

In this case, you bought a lock to protect the bike, and it worked, which is great.