chore: use pytest-insta to manage snapshots

[JamesGuthrie]

This change uses pytest-insta to manage snapshot tests.

pytest-insta integrates seamlessly with pytest. When the output of a snapshotted test changes, the test fails.

If you're expecting the changes and want to update the snapshot, run pytest --insta update.

To review the changes on an individual basis, run pytest --insta review.

To view the new snapshot output, run pytest --insta record, this outputs the path to the file containing the new snapshot:

> pytest --insta record -k test_contents
=========================== test session starts ============================
platform linux -- Python 3.10.15, pytest-8.3.2, pluggy-1.5.0
rootdir: /home/james/workspace/pgai/projects/extension
configfile: pyproject.toml
plugins: insta-0.3.0, anyio-4.7.0
collected 112 items / 111 deselected / 1 skipped / 1 selected

tests/contents/test_contents.py .                                    [100%]

================================ SNAPSHOTS =================================
RECORD .pytest_cache/d/insta/tests/contents/contents__contents__pg17.txt

NOTICE 1 snapshot to review
=============== 1 passed, 1 skipped, 111 deselected in 0.39s ===============