Minecraft – make any librarian give a mending book

The only items that can be enchanted are Golden Swords from Zombie Pigmen and Bows from Skeletons.

The swords, if enchanted, have their enchantment chosen randomly as if it were a level 5 enchantment from an enchanting table. The following enchantments are therefore possible:

Sharpness I–III
Smite I–IV
Bane of Arthropods I–IV
Knockback I–II
Fire Aspect I–II
Looting I–II

Bows, if enchanted, will always have Power I.

Villagers are all randomly assigned a profession on spawning, and unfortunately, this is an even 1/6 chance for each one. For specifically Farmers (so you can trade) this is then further broken down to the 4 different brown coat professions, ultimately making the chance of a Farmer spawning only 4.17%. Source.

Fortunately, it would appear that baby villagers are also assigned their professions at random, so it is entirely possible to create a Villager Breeder in order to find the Villager you are after, or simply boost the amount of doors you have in your village. Even by simply creating a 3 wide covered platform with in the village, and fill that with doors will greatly increase your villager breeding rate.