Add test to verify all Jupyter kernels in images launch successfully

[coderabbitai]

Follow-up from PR #1218
(reference comment: #1218 (comment)).

Implement a pytest under tests/containers/ that:

• Runs each built notebook image (e.g. datascience, pytorch, tensorflow, …) via podman run.
• Lists all installed Jupyter kernels with jupyter kernelspec list --json.
• For each kernel, attempts to launch it using jupyter_kernel_test (from PyPI).
• Fails if any kernel cannot be started.
• Integrates the test into the existing container-test job so the CI pipeline blocks on broken kernels.

Owner: @jiridanek