Test: JupyterLab settings registry/schema is error-free

[coderabbitai]

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

Implement a test that:

• Fetches /api/settings from a running JupyterLab server.
• Checks for schema errors, migration issues, or error fields in the response.

Sample code:

import requests

def test_jlab_settings_schema():
    r = requests.get("http://localhost:8888/api/settings")
    assert r.status_code == 200
    data = r.json()
    assert "error" not in data
    # Optionally: check for known schema/migration error keys

Owner: @jiridanek