Insurance companies are free to come to their own conclusions.
Leave this to your insurance company. You are out your deductible no matter who the other company says is at fault.
What would you hope to get from small claims court? Your deductible? If you go to court, the other side may be able to deny any payment at all and you may lose any potential settlement. I'd leave it to your insurer.
But if you need it, there is this:
22517. No person shall open the door of a vehicle on the side
available to moving traffic unless it is reasonably safe to do so and
can be done without interfering with the movement of such traffic,
nor shall any person leave a door open on the side of a vehicle
available to moving traffic for a period of time longer than
necessary to load or unload passengers.
Though, I suspect the other party will argue that the door was open for a long time and had you been paying attention you could have avoided the collision.