🐛 Fix bug regarding assumed default work type

[jeremyf]

This commit includes two things:

1. Favor Bulkrax::EntrySpecHelper provided by Bulkrax (for which the removed code was a prototype).
2. Use the existing spec to determine what the default work type should be (instead of relying on a random assignment based on the first element of the registered curation concerns)

And from this commit, we have a passing test!

Related to:

• https://github.com/scientist-softserv/adventist_knapsack/issues/324
• https://github.com/samvera-labs/bulkrax/pull/726
• https://github.com/samvera-labs/bulkrax/pull/719
• https://github.com/samvera-labs/bulkrax/pull/742
• https://github.com/samvera/hyku/pull/1811