Adapts GgenIgniter.Query.Qlever (real, remote QLever HTTP) to
GgenIgniter.Engine.
:gno and :tesla are both optional: true in mix.exs (see the comment
there). Neither is referenced here at compile time (no struct patterns, no
aliases into Gno/Tesla) -- every reference is a plain remote call or a
bare atom (Application.ensure_all_started(:tesla)), so this module always
compiles regardless of whether a consumer has those deps. prepare!/2 does
add real Code.ensure_loaded?/1 runtime guards, though: without them, a
consumer missing :gno/:tesla would still hit --engine qlever and get a
raw MatchError (from Application.ensure_all_started(:tesla) returning
{:error, ...}) instead of a clear, actionable message -- and would hit that
confusing error before ever reaching GgenIgniter.Query.Qlever.load_store!/2
(whose own stub, compiled when :gno is missing, raises the same message).