Fix off-by-one error in fulfillment code

[tomwheeler]

What was changed

This fixes an off-by-one error (selection of an array slice omits the final element) in the Activity that creates a fulfillment.

Why?

When submitting an order for two items, I found that there were two fulfillments, as expected. However, I also found that the second one was empty.

Checklist

1. Closes: N/A

2. How was this tested:
   I tested this manually, observing that the second fulfillment was empty (before the change) and that it contained the expected content (after the change). I hope to create an automated test to validate the desired behavior, but wanted to submit a draft PR with the fix first.

3. Any docs updates needed?
   No

[robholland]

For the no items case, the activity should cleanly return an empty result. That should be covered by a test. From a business point of view, the workflow should reject orders with no items, but the activity code should still handle that case.

[robholland]

Approved because I'm fine for the new edge case and test to be in a different PR if you prefer.

[tomwheeler]

I just pushed a couple of commits that simplify the two tests and handle the zero-items case (along with a test to verify the behavior).