Trusting notebook in RTC mode leads to File Changed dialog

[krassowski]

Description

Regardless of fixes in #14345 the trust status is not set for empty (1 code cell) notebooks on RTC, and an attempt to Trust notebook ends up with File Changed dialog.

Reproduce

1. Spin up RTC setup on binder
2. Open command palette
3. Choose "Trust Notebook"
4. Click Trust
5. See File Changed dialog

Expected behavior

• whatever causes the trusted metadata to be lost on code cells when RTC is active gets ironed out.
• trusting notebook works the same on RTC as it works locally, without dialog.

Context

• Browser and version: latest Chromium family
• JupyterLab version: 4.0.0b1