Test: All JupyterLab extensions are enabled and activated

[coderabbitai]

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

Implement a test that:

• Runs jupyter labextension list --json in the container.
• Asserts all expected extensions are present, enabled, and activated (not just installed).
• Optionally, queries /lab/api/extensions for runtime status.

Sample code:

import json, subprocess, os

def test_jlab_extensions_enabled():
    cmd = ["podman", "run", "--rm", os.environ["IMG"], "jupyter", "labextension", "list", "--json"]
    out = subprocess.check_output(cmd, text=True)
    data = json.loads(out)
    for ext, meta in data.get("installed", {}).items():
        assert meta.get("enabled", True), f"{ext} not enabled"
        assert meta.get("is_ok", True), f"{ext} not healthy"

Owner: @jiridanek