Fix/server version endpoint

[khalsz]

No description provided.

[olivhoenen]

Thanks @khalsz, it looks good and makes sense to me, but we still need to address backward compatibility somehow. See below the behavior with a CLI client based on this PR and our production server based on 0.15.1 (same with dev):

(venv) [hoeneno@98dci4-srv-1001 SimDB]$ (fix/server-version-endpoint) simdb remote test 
Error: 'server_version'

We will need to have a strategy there, hereafter are some thoughts from copilot:

Suggested fixes

1. Make get_server_version() tolerant: try the new root endpoint first, and if
   server_version is missing or the request fails, fall back to the versioned root
   (self.get("", authenticate=False)); use .get("server_version") instead of
   [...] so a missing key degrades gracefully (e.g., returns None/"unknown")
   instead of raising.
2. Alternatively, keep server_version in both locations on the server for one
   deprecation cycle before removing it from the versioned index.
3. Add an integration test that builds a server response missing server_version at
   one of the two locations and asserts RemoteAPI construction still succeeds —
   current tests mock get_server_version/get_api_version directly, or use a Flask
   test client that's always fully up to date on both sides, so they don't catch
   this cross-version scenario.

Other notes

Dropping the extra get_api_version() round-trip in favor of the already-negotiated
selected_version string looks safe and backward compatible — selected_version was
already validated by select_api_version() against the server's advertised
endpoints.