Fantasy book with self fulfilling prophecy, main character has red hair, (and maybe wears a green dress in the cover art)

Hi all, I’ve been trying to figure out what this book was and I’ve described it to my parent and multiple friends and no one knows what it is. It’s been driving me crazy so I hope someone here can help. I read this in elementary school so it can’t be any books released after 2012/2013.

I only remember the ending of this book so if you don’t want spoilers for stop here.

What I remember is that it was kinda maybe like medieval fantasy setting. There’s a war brewing and a prophecy has been made that the war will end when a soldier with a fire halo (and maybe a white horse?) rides into battle and brings peace. The ending of the book is the heroine fixing her red hair into a halo and getting on her horse and riding between the two armies to bring peace to her kingdom.

For some reason I have the vague memory of the character on the cover art wearing a green dress, but I might be confusing it with the Ella Enchanted book cover or maybe Merida? Idk, but it’s really driving me crazy!

This is all I remember if you know this book please tell me the name!