yasova dragonclaw steals a creature until end of turn. I have sundial of the infinite. When Yasova's steal a creature ability goes off on the end step, I respond to it by using sundial of the infinite. Do I now get to keep the creature I stole when sundial's ability resolves? My thinking is, that creature has already had it's end of turn trigger go on the stack. Sundial then ends the turn before the end step would finish. Can I permanently steal a creature in this way?
No. Yasova's control change effect ends in the cleanup step, it is not something you can respond to. It is not a trigger. Ending the turn just moves the game to the cleanup step, not beyond it, where the control change effect then ends like it would normally do.
715. Ending the Turn
715.1. Some cards end the turn. When an effect ends the turn, follow these steps in order, as they differ from the normal process for resolving spells and abilities (see rule 608, “Resolving Spells and Abilities”).
715.1d The current phase and/or step ends. The game skips straight to the cleanup step; skip any phases or steps between this phase or step and the cleanup step. If an effect ends the turn during the cleanup step, a new cleanup step begins.
Yasova Dragonclaw's control changing ability doesn't have a trigger that goes off on the end step. Much like effect like Giant Growth the control changing aspect continues until the clean up step at end of turn with no one able to respond. Sundial of the Infinite does not affect this interaction because 'until end of turn' effects still where off like normal when you end the turn with its effect. It's the last line of the reminder text on sundial of the infinite.