fix: remove admin bypass from server access helpers (#9933) (#10006)

In server mode, administrators were auto-granted visibility into every
user's private server groups and servers via four _is_admin() bypasses
in server_access.py. This made the Object Explorer show a separate
top-level "Servers" entry per user when logged in as admin, exposing
private connections the admin should have no access to.

The Administrator role in pgAdmin governs management of pgAdmin itself
(users, preferences) — it is not intended to inherit other users'
database credentials and connection state. Cross-user visibility
requires explicit sharing (Server.shared=True), same as for any user.

Remove the admin bypass from get_server, get_server_group,
get_server_groups_for_user, and get_user_server_query. Drop the now-
unused _is_admin() helper. Update docstrings to make the policy
explicit.

Add a regression test (admin attempts to fetch a non-admin user's
private server group → expect HTTP 410). The original isolation test
only covered non-admin → admin, which is why the regression
introduced by 9a76ed8 was not caught.