fix: disallow nesting of part_of_a_transaction

[Giggitycountless]

What changed

Add to the atomic call inside part_of_a_transaction so that nesting is explicitly disallowed.

Why

As noted in #150, just as it doesn't make sense for transactions to be nested, it doesn't make sense for tests to simulate nesting of parts of transactions. Adding durable=True causes Django to raise a RuntimeError if part_of_a_transaction is called inside an existing atomic block, preventing accidental misuse.

Changes

1. src/django_subatomic/test.py: Add durable=True to transaction.atomic() call
2. tests/test_db.py: Add TestPartOfATransaction class with 3 tests:
   • Nesting part_of_a_transaction inside another raises RuntimeError
   • Nesting part_of_a_transaction inside an existing atomic block raises RuntimeError
   • part_of_a_transaction correctly creates a transaction when used standalone
3. CHANGELOG.md: Document the change in the Unreleased section

How to test

python -m pytest tests/test_db.py::TestPartOfATransaction -v

All 3 new tests pass, and the full test suite (86 passed, 2 skipped) is green.

Fixes #150

[meshy]

Thank you for contributing this! I realised last night that this blocks another ticket I've been working on, so I was going to pick this up this morning, but I see you've beaten me to it!

There are a couple of minor changes I'd like to see to this, but seeing as I was going to work in this this morning, I'm going to merge this now and apply my tweaks on top.

Thanks again!

[meshy]

Actually, this has highlighted a minor issue in the CI pipeline. I'm going to address that before getting around to merging this. I may as well list the tweaks I'm going to make on this PR now, so I don't forget (and in case you happen to get to them first).

[meshy]

Thanks again @Giggitycountless. This has now been merged as part of #153