docs: describe create_savepoint session instead of removed CustomAsyncSession

[lesnik512]

AGENTS.md was out of date with the code in two places:

• Persistence referred to a CustomAsyncSession that overrides close(). That class is gone. create_session in app/resources/db.py now builds a plain AsyncSession with join_transaction_mode="create_savepoint", which does nothing in production and, in tests, gives the session its own savepoint so the outer transaction survives commits.
• Test isolation said the db_session fixture "starts a SAVEPOINT". It doesn't: tests/conftest.py only begins the outer transaction. The savepoint comes from each session's join_transaction_mode.

The new wording follows fastapi-sqlalchemy-template/AGENTS.md.