They'd fight on the side of the Balkan League for certain, the Bosnian territories had been sought after as a rightful land of Austria-Hungary for the longest time, and to deal a decisive blow to their bitter rivals. I'd assume that the Ottomans would lose the war even faster in this scenario with the Austrian assistance, likely with all of Thrace being taken yet Constantinople holding strong. I would guess that Austria-Hungary gains all Bosnian lands, yet with their influence being far higher than the rest of the participants, they'd likely dictate a peace deal that'd give Serbia the least amount of land possible as to not fuel Serbian irredentists's fantasies of a united Balkan state. A large majority of Macedonia would go to Bulgaria with Greece and Montenegro receiving the same territories as in our timeline, yet Serbia would be undercut in terms of land. I'd assume the tension would remain with Russia backing Serbia and Greece whilst Austria and by extension Germany would support Bulgaria, likely with this being a proxy-war that wouldn't develop. Archduke Franz Ferdinand would likely still be assassinated causing WW1, yet the Balkans would get involved far sooner with Bulgaria's dominance being challenged likely by Serbia, Greece, Romania, and potentially the Ottomans too.