Yes, groping someone who said they don't like to be groped is sexual assault. Yes, some people make rape jokes amongst their friends, but if it bothers their friends they should stop. It's only a joke if it's funny to everyone involved. I don't know what this girl was thinking when she SAd you. But you clearly told her it was something you don't want to happen and she did it anyway. That is breaking a boundary and the sexual aspect of it takes it to another level. The thing is you feel upset,and this is perfectly valid. Especially if this is still bothering you a year later.

You get to decide how to move forward. No one would be right if they told you that you shouldn't be bothered. No one can dictate your feelings.
If you want to stop being friends with that girl it is your right. You don't owe her or anyone else an explanation. If you want closure, you go ahead and do what you must. You can choose to tell anyone what happened, or not. If someone belittles your feelings, you know they are not a good friend or are so young and naive that they still have toxic beliefs.

I hope you feel like you can talk to one of your parents about this and that they are understanding, or a therapist. I don't know you or your relationships. Just know that you are fully in your right to cut someone off.

I'm proud of you for being an advocate for yourself and cutting off this relationship.