Test: JupyterLab REST API endpoints return valid responses

[coderabbitai]

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

Implement a test that:

• Starts the container and waits for JupyterLab.
• Calls /api/contents, /api/settings, /api/workspaces.
• Asserts HTTP 200 and valid JSON, with no error fields.

Sample code:

import requests

def test_jlab_api_health():
    base = "http://localhost:8888"
    for path in ["/api/contents", "/api/settings", "/api/workspaces"]:
        r = requests.get(base + path)
        assert r.status_code == 200
        data = r.json()
        assert "error" not in data

Owner: @jiridanek