What if Austria-Hungary mobilizes and declares war on Russia to force Russia to back down from the Treaty of San Stefano?*
If Germany feels compelled to support Austria-Hungary (because it can't let Austria-Hungary collapse) with the support of arms or the threat of arms, what does France do?
Does France quickly and unhesitatingly move to support Russia to gain an ally?
The Britain factor:
Also note that Britain, if anything, will be even *more* supportive of Austria-Hungary than Germany will be. That's a complicating factor for France. I would think that the British would fight on the Austro-Hungarian side against Russia, or at the very least, take all measures short of war to help A-H. While down the line, if the result is an Ottoman-Habsburg-Hohenzollern beat down of Russia, Britain may well come to find that situation distressing for balance of power reasons. But, I think it would take more than a few months for Britain to back away from its anti-Russian orientation.
*if you wish, we could consider an alternate PoD, where the Austro-Hungarians intervene earlier in the Russo-Turkish war, perhaps while the Turks are still fighting hard at Plevna. I chose post San Stefano as the initial PoD however because that's when Russia's behavior really started to try the patience of other European powers the most.