chore: scope postgres_container fixture to class

The postgres_container fixture was scoped to 'session', effectively all
tests. This meant that the state in the database was shared across all
tests. This is good for speeding up tests, because the container must
only be constructed once, but bad for tests being independent (because
they rely on shared state).

This change scopes the postgres_container to 'class', and adds a new
class to group tests which can be run together with the same database
state. This balances the tradeoff between "every test must build
everything from scratch" and "tests are very slow".

projects/pgai/tests/vectorizer/conftest.py

@@ -46,7 +46,7 @@ def vcr_():
     )
 
 
-@pytest.fixture(scope="session")
+@pytest.fixture(scope="class")
 def postgres_container():
     extension_dir = os.path.abspath(
         os.path.join(os.path.dirname(__file__), "../../../extension/")