remove random selection policy — consumers can shuffle client-side

Plano should only handle ranking that requires server-side data
(cost metrics, latency). Random shuffling is trivial for callers.

crates/common/src/configuration.rs

@@ -109,7 +109,6 @@ pub enum StateStorageType {
 pub enum SelectionPreference {
     Cheapest,
     Fastest,
-    Random,
     /// Return models in the same order they were defined — no reordering.
     None,
 }