the dog barking is obviously an inconvenience and an issue you’ll need to address with your neighbour - something you clearly recognise. And trying to speak to them is a good idea. As they should have also if they are your immediate neighbour rather than send a passive aggressive text.
However I don’t agree at all that throwing the chocolate over is either innocent or not a police matter - poisoning someone’s pets is a vile act and, more importantly, a criminal offence. It’s no more a waste of police time than theft of a car or criminal damage to property.
Chocolate being poisonous to dogs is pretty commonly known. We don’t even own a dog and are well aware of how dangerous sufficient quantities of theobromine (the active ingredient in chocolate that is poisonous to dogs) can kill a dog. Similarly raisins, green algae and a bunch of other things in sufficient amounts...the fact that a dog ends up ok will be down to the dose - size and weight of dog vs concentration of theobromine in the chocolate. High quality dark chocolate will have more of it. Milk chocolate less.
So for a small dog like a terrier you don’t need as much for it to make the dog sick or to kill it, whereas a big heavy lab might get away with a lot more.
The suggestion that the neighbour didn’t know about chocolate is unconvincing - the only reason to throw the chocolate over, particularly if the dog is barking, is as some sort of treat or as a poison in the hope it might make them sick. The former is highly unlikely given he obviously doesn’t feel positively towards your pet; which leaves the latter. It may be that he didn’t intend to kill your dog but to make it sick but either way, completely inappropriate, reckless and potentially criminal.
So I would go round, speak to them, tell them you’re doing your best about the noise but press them on the chocolate. I would consider calling the police or so as not to escalate at least telling them you’re giving them the benefit of the doubt about the chocolate in an attempt to move things forward but that you take it very seriously.
You aren’t “in the wrong” - we all have to try to live with one another. Like many, we have kids who play in the garden and can be noisy at times when left unsupervised For too long; similarly other neighbours have barking dogs, crying babies or loud house parties. We all try to accommodate each other. Your neighbour should have done the same and spoken to you - you will have to be the grown-up.
But don’t be afraid to contact the police about this as you may end up with a dead dog - a willingness by your neighbour to resort to that kind of anti-social and criminal behaviour so quickly is not a good sign.