test: fail db_session teardown when the outer transaction was closed

[lesnik512]

Closes #23

What changed

db_session's teardown in tests/conftest.py used to roll back only if connection.in_transaction():. If something had committed the outer test transaction (for example a session built without join_transaction_mode="create_savepoint"), the rollback was skipped, the test's writes stayed in the database for later tests, and nothing said so.

Teardown now checks transaction.is_active on the outer transaction itself:

• If it is still active, it is rolled back, same as before.
• If it is not, pytest.fail reports that the outer transaction was committed instead of nesting a savepoint and points at join_transaction_mode="create_savepoint". The test shows up as an error in teardown.
• Closing the connection, disposing the engine and resetting the DI override now sit in an inner finally, so they run on the failing path too.

I checked transaction.is_active instead of keeping connection.in_transaction() on purpose. After the outer transaction is committed, any later statement on that connection autobegins a new transaction, so in_transaction() would be true again even though the original writes were already committed. Looking at the transaction object covers that case as well.

create_session and the production code are unchanged.

Test

tests/test_db_session_fixture.py uses pytester (enabled through pytest_plugins = ["pytester"] in that module) to run an inner pytest session in-process. The inner file imports the real db_session, di_container and app fixtures from tests.conftest, so it runs against the same container database, and coverage of the teardown branch is attributed to tests/conftest.py. The inner session:

1. commits the outer transaction through a join_transaction_mode="control_fully" session that runs a statement on db_session.bind;
2. in a second test, asserts that the connection captured in step 1 is closed, which shows cleanup still ran after teardown failed.

The outer test expects 2 passed and 1 error, plus the failure message.

Test results

• Before the fix (original conftest.py, new test): 1 failed. The inner run reported 2 passed, errors: 0 where 1 was expected, so teardown was silent.
• After the fix: tests/test_db_session_fixture.py gives 1 passed.
• Full just test: 111 passed in 37.53s, coverage 100.00%. No existing test trips the new check, which confirms that nothing in the suite closes the outer transaction.
• just lint: eof-fixer and ruff format left all files unchanged, and ruff check and ty check both reported "All checks passed!".

The order-dependent isolation tests in tests/test_main.py are #19 and are not touched here.