MerchantRecipe result setter

Is your feature request related to a problem?

It seems needlessly complicated to register a fresh merchant recipe and replace the original when all you want to change is the result item.

Describe the solution you'd like.

I think just adding this to paper-api/src/main/java/org/bukkit/inventory/MerchantRecipe.java oughta do it. Haven't tested it, though.

    public void setResult(@NotNull ItemStack item) {
        Preconditions.checkArgument(!item.isEmpty(), "Result cannot be an empty itemstack.");
        result = item.clone();
    }

Describe alternatives you've considered.

The recommended approach I've seen is to unregister the recipe and register a new one in its place, which requires copying in all the old values and doesn't seem like a very robust solution.

Other

I've poked around and haven't seen any discussion about why this would be a bad idea, so I'm assuming it was just never added. I don't see a reason why the result has to be immutable, but this isn't my code base so I can't be sure.

Start with paper-api/src/main/java/org/bukkit/inventory/MerchantRecipe.java and review the existing result handling and validation around MerchantRecipe. Confirm the proposed setter preserves the stated non-empty result behavior, then run the repository's relevant API checks; done means callers can replace the result without unregistering and recreating the recipe.