Summer wouldn't do anything if Mab attacked the White Council in response to an attack on her Knight. They exist to stop Winter from preying on innocent mortals but those who embroil themselves in the affairs of the courts lose that protection. Attacking Harry without provocation would definitely be seen as them deliberately involving themselves. Mab may not be able to stop them or avenge Harry right away, but eventually she'd balance the scales like she did with Nicodemus in Skin Game.