Detect inconsistent initial steps in yjs and require page reload

[max-nextcloud]

Is your feature request related to a problem? Please describe.
In case two users start a new yjs session at the same time they need to initialize it with the initial ydoc content to create a consistent world view. #5589 tackles this. If for some reason the two clients come up with a different initial doc state we end up with a "split brain" situation - where both work on a similar yet different document and fail to sync fully.

Planned solution

• Push the doc state as soon as we have it to reduce the timespan in which conflicts may occure.
  • Use a new endpoint to save the document state only without the autosave content.
  • Use a separate wrapper in the SessionApi on the client side.
  • Store documentState but no autosaveContent to avoid writing unchanged documents.
• In case a conflicting documentState already exist:
  • Log the information this happened.
  • On the client handle the error response with an appropriate message and ask the user to reload the page.
• backport to stable28 - as that's how far Always initialize with the same yjs document if no state is present #5589 was backported.

Tasks

• New API endpoint that only stores the initial document state.
• Wrapper in SessionApi for that endpoint.
• Test the API endpoint from cypress api tests.
• Send the initial document state to the server as soon as we have it.
• Handle errors on the client side.

Acceptance Criteria

• If one client connects a following client will receive an initial yjs document state shortly after (< 5sec.)
• If two clients connect within the same timeframe and the initial yjs document state matches they continue without interruption.
• If two clients connect within the same timeframe and the initial yjs document state differs one gets to continue - the other one displays an error message that instructs the user to reload the page.

Open questions

• Does this interfere in somehow with
  • read only sessions
  • reconnecting
  • reconnecting after a session cleanup