This bothered me too. Not just concerning Avraham, but by many of the situations mentioned in 'Mi Sh'ana', we never find the people mentioned actually praying or asking anything of Hashem.
The answer I've told myself is based on an Or HaChaim in Shmos chapter 2 vs 23-24. The Torah tells us that the cries of the Jews from their work went up to Hashem And Hashem heard their cries. The Or HaChaim explains 'it was not the case that they prayed to Hashem to save them,but they cried from the pain like a person crying from his pain. And the Torah tells us that this crying went up before Hashem etc'.
It seems that Hashem is able to equate a person's cries of pain to a prayer when He sees fit. Now, although we don't find Avraham or any of the others in Mi Sh'ana actually crying out, the fact of the matter is they would have liked the situation to be different, otherwise this would not be a test. Hashem's intervention is called a response, even if there was no actual prayer.
I am aware Rashi in vs 6 says Avraham went with desire and happiness to slaughter his son, but this was a description of his actions And convictions based on Hashem's command, not what he personally would have chosen to be. Otherwise it would not have been a test as I mentioned before.