Select the Sauté setting on the Instant Pot and heat the oil for 2 minutes. Using tongs, add the chicken in a single layer and sear for about 2 minutes, until lightly browned. Flip the chicken and sear for 2 more minutes, until lightly browned on the second side, then transfer to a plate. Add the garlic, onion, mushrooms, and the remaining 1/4 tsp salt and sauté for about 4 minutes, until the onion begins to soften and the mushrooms begin to wilt and give up their liquid. Add the Italian seasoning, wine, and tomatoes and stir to combine, using a wooden spoon to nudge any browned bits from the bottom of the pot. Let the wine come to a simmer, then add the chicken breasts in a single layer on top.