Man I am sorry to hear that. What is the owner's liability there in France? In the US the owner would be cited; the dogs taken in to make sure they don't have rabies; and the owner would be responsible for your medical bills.
Dogs by nature are pack animals so I am not surprised by the other dog getting involved. If you ever notice when you see dogs from a distance that are chasers they size you up in terms of speed and how or what line they need to take to intercept you. Comes from hundreds of thousands of years of evolution and instinct when hunting. Can't blame a dog for being a dog............the OWNER is the Jackwagon who should maintain their pets. So to give advice on "What could I have done different?" Not sure if anything given your in a climb and vulnerable to not being able to accelerate and blast out of there. I have found on flats when they size you up you have one of two things to do................sprint like a mad man and drop them or time your cadence to their intercept and then either brake hard then get back on it or all out sprint to make them miss you. No doubt about it we are vulnerable to dogs on bikes. And not just vicious dogs but those who like to chase.
Hope you did not suffer real bad injuries. I skipped over all the post after yours to comment.