fix(ui-models): stop the shared hint contradicting a filled-in field

The line under the create form names the one blocked gate worth naming,
and its fallback arm reads "no models yet". An unmet Provider ID gate fell
through to that arm, so a card with two models listed right above it was
told it needed one. The key gate was already excluded for this reason; the
route gate was assumed excluded because its field explains itself, and was
not.

Tightening the route rule in the previous commit is what made this easy to
hit — a digit-leading id now fails the gate — but the fallthrough predates
it and fires for an empty or taken id just the same.

packages/client/ui-models/src/client/CustomProviderCard.tsx

@@ -112,14 +112,17 @@ export function CustomProviderCard(props: CustomProviderCardProps): ReactNode {
   const ready = route.length > 0 && !routeInvalid && !routeTaken
     && baseURL.length > 0 && models.length > 0 && modelFailure === undefined
     && keyFailure === undefined
-  // The one blocked gate worth a line under the form. The route id is omitted
-  // because its own field already explains itself, and a satisfied card says
+  // The one blocked gate worth a line under the form. A satisfied card says
   // nothing at all rather than printing an empty paragraph.
   const hint = failure !== undefined || ready
     // The key field prints its own failure directly beneath itself, so a card
     // blocked only by the key stays silent here rather than answering with the
     // next unmet gate — which is satisfied, and reads as a second, false fault.
     || keyFailure !== undefined
+    // Same for the route id, and it must be tested rather than assumed: the
+    // fallback arm below reads "no models yet", so an unmet route gate used to
+    // fall through to it and contradict the filled-in list right above.
+    || route.length === 0 || routeInvalid || routeTaken
     ? undefined
     : baseURL.length === 0
       ? t('customNeedsBaseUrl')