fix(composite): pass the requested model through when a prefix route leaves upstream_model empty

Composite model routes support a match_type: prefix mode with an optional
upstream_model. Leaving upstream_model empty is meant to pass the concrete
requested model through to the upstream, but every request matching the prefix
collapsed to the route's public_model instead. A route with
public_model=deepseek-v4, match_type=prefix and an empty upstream_model
forwarded both deepseek-v4-flash and deepseek-v4-pro upstream as deepseek-v4.

Root cause: normalizeCompositeRouteInput backfilled an empty upstream_model with
public_model at save time, so CompositeRouteResolver.Resolve never reached its
"upstreamModel == '' -> concrete requested model" fallback.

- Skip the backfill for prefix routes only; an empty upstream_model stays empty
  and Resolve passes the concrete requested model through.
- exact routes keep the backfill (model == public_model, so persisted/display
  contract is unchanged).
- Explicit upstream_model still forwards that fixed model in every mode.
- Frontend: add an upstream_model hint (zh/en) documenting empty = passthrough.

frontend/src/views/admin/GroupsView.vue

@@ -3809,6 +3809,9 @@
                 class="input"
                 placeholder="gpt-5"
               />
+              <p class="mt-1 text-xs text-gray-500 dark:text-gray-400">
+                {{ t("admin.groups.compositeRoutes.upstreamModelHint") }}
+              </p>
             </div>
 
             <div>