[tonyandrewmeyer]

Backport of the move from pytest-operator and python-libjuju to Jubilant, so that the 2.23 maintenance branch no longer depends on either.

Cherry-picked from main, in order:

• chore: jubilant smoke tests #1958 (smoke tests to jubilant-backports)
• ci: add Juju 4/beta to the smoke test matrix #1963 (drop the juju dependency and add Juju 4 to the smoke matrix)
• docs: update the httpbin charm to jubilant #1987 (httpbin example charm)
• docs: update zero to hero to jubilant #1988 (zero-to-hero tutorial and the k8s example charms)
• docs: update how-to to feature Jubilant (1/2) #2000 and docs: update how-to to feature Jubilant (2/2)  #2004 (how-to guides and the testing explanation)
• ci: ignore juju/4 timeouts #1998 (ignore Juju 4 on microk8s timeouts in the smoke matrix)
• ci: use Juju channel 4/stable in Ops smoke tests #2190 (Juju 4/stable rather than 4/beta in the smoke matrix)

Adjustments for the 2.x branch:

• The Python 3.8 and 3.9 environment markers that chore: jubilant smoke tests #1958 removed from pyproject.toml are kept, as is the 20.04 base in the smoke test parametrisation.
• The smoke tox env injects ops-2.*.whl rather than ops-3.*.whl.
• uv.lock was regenerated rather than merged, which drops pytest-operator, python-libjuju, pytest-asyncio and their transitive dependencies.
• The Jubilant intersphinx entry is in docs/custom_conf.py, since this branch still has the older starter-pack layout.
• The tutorial's "review the final code" links stay on the juju-sdk-tutorial-k8s repository, which is what the other chapters on this branch link to.

[james-garner-canonical]

Thanks, I didn't read all the changes thoroughly, but this looks structurally sound.