test(testkit): use one pullable test image with the tools ported tests need #5352

User Story

I'm moving e2e/rust tests onto testkit and running conformance against gateways other than the ones our lanes set up. I need one test image that any gateway can pull and that has the tools the ported tests use. Then no lane has to build or load a test image, and a gateway outside this repo can run conformance as is.

Description

Today sandbox tests use two different images:

• openshell/e2e-python:dev, built from e2e/python/Dockerfile.workload. It has system Python, a uv Python and virtualenv, curl, git, openssl, requests, google-auth, cloudpickle, and a non-root sandbox user. It isn't published, so it exists only where a lane builds it (the Docker and Podman e2e/rust scripts, mise run e2e:workload:build) or loads it (the tmachine installers).
• ghcr.io/astral-sh/uv:0.12.17-python3.12-trixie-slim@sha256:…, a public image pinned by digest. It has the Python standard library only: no curl, git or openssl, and no non-root user.

As of #5318, testkit::python_image() defaults to the public uv image, so conformance works against any gateway. OPENSHELL_TEST_PYTHON_IMAGE overrides it. That covers the current tests, which use only the standard library. Many e2e/rust tests still to be ported (#5281, #5297) use curl, git, openssl or requests, and would fail on the uv image.

Pick one image to replace both:

1. A trusted public image with more tools. It should be maintained by a known publisher, pinned by digest, and include Python plus curl, git and openssl. That means nothing for us to publish or maintain.
2. Our own test image, published. Publish e2e/python/Dockerfile.workload (or a slimmer version) to ghcr from CI, pinned by digest in testkit. We control its contents, at the cost of a publish step and keeping its dependencies up to date.

Context

• tests/testkit/testkit/src/image.rs: python_image(), PYTHON_BINARIES.
• The legacy harness has the same split: E2E_WORKLOAD_IMAGE in e2e/rust/src/harness/sandbox.rs.
• tmachine builds and loads e2e-python:dev (tests/tmachine/artifacts.nix, config.nix, and the openshell_gateway role and openshell-k3s.yaml playbook). Those Ansible comments say the testkit uses it, which is no longer true. The Podman driver e2e playbook (drivers/podman/e2e.yaml) still uses it directly.
• openshell/e2e-python:dev has no registry in its name, so a pull of it resolves to Docker Hub. Any default or override must name a registry and a digest.
• The image must work on every driver without a named workload user in the test policy. Podman rejects a policy user the image doesn't have.

Definition of Done

• One image is chosen, with the reasoning recorded here.
• testkit::python_image() defaults to it, pinned by registry and digest. PYTHON_BINARIES covers its Python paths.
• Conformance passes on every conformance row and host lane with no OPENSHELL_TEST_PYTHON_IMAGE set.
• tmachine and the host lanes no longer build or load an image for testkit suites, or a remaining use is documented.
• If we publish our own image, CI publishes it and something keeps its base and packages updated.
• tests/testkit/README.md and TESTING.md describe the image and the override.