Ecommerce Plugin - Expose manual refreshCart and updateCart in useCart

[teastudiopl]

I’m currently adding products to the cart by fetch request because the standard addItem method has built-in validation for product uniqueness conflicts with my use case: the same product as new line in cart.

To make this workflow smoother, could you:

• Expose a way to manually refresh the cart state, so I can re-sync the UI whenever I modify cart items directly.
• Provide access to updateCart from useCart - this would allow updating the cart programmatically while keeping the state consistent.

This would allow more flexibility for custom cart logic without being blocked by addItem’s uniqueness validation.

[teastudiopl]

I've created PR for this, #14767