Add JupyterLab server smoke test (HTTP 200, no JS errors)

[coderabbitai]

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

Implement a test that:

• Starts the notebook image (e.g. with podman run -p 8888:8888 ...).
• Waits for JupyterLab to be ready.
• Fetches /lab (or /lab?reset) and asserts HTTP 200.
• Optionally, inspects logs for JavaScript errors (look for "Uncaught" or "TypeError" in the browser console output).

Sample code (Python, using requests):

import os, subprocess, time, requests

def wait_jlab_ready(url, timeout=60):
    for _ in range(timeout):
        try:
            r = requests.get(url)
            if r.status_code == 200:
                return True
        except Exception:
            pass
        time.sleep(1)
    raise RuntimeError("JupyterLab did not become ready")

def test_jlab_server_smoke():
    # Start container in background (adapt as needed)
    proc = subprocess.Popen([
        "podman", "run", "--rm", "-p", "8888:8888", os.environ["IMG"]
    ])
    try:
        wait_jlab_ready("http://localhost:8888/lab")
        # Optionally: fetch logs and check for JS errors
    finally:
        proc.terminate()

Owner: @jiridanek