refactor: address code review feedback for client route type safety

- Replace 'type in routes' guard with direct property access + truthiness check
- Remove unnecessary 'as string' type assertion
- Add comments clarifying discovered vs client-specific route handling

Agent-Logs-Url: https://github.com/objectstack-ai/spec/sessions/283e383b-284b-40fe-af98-c11f61a4c8a8

Co-authored-by: xuyushun441-sys <255036401+xuyushun441-sys@users.noreply.github.com>

Author: Copilot

packages/client/src/index.ts

@@ -1686,13 +1686,15 @@ export class ObjectStackClient {
    * ObjectStack uses standard conventions: /api/v1/data, /api/v1/meta, /api/v1/ui
    */
   private getRoute(type: ApiRouteType): string {
-    // 1. Use discovered routes if available
+    // 1. Use discovered routes if available (only for ApiRoutes keys, not client-specific keys)
     const routes = this.discoveryInfo?.routes;
-    if (routes && type in routes) {
-        return routes[type as keyof ApiRoutes] as string;
+    if (routes) {
+        const key = type as keyof ApiRoutes;
+        const discovered = routes[key];
+        if (discovered) return discovered;
     }
 
-    // 2. Fallback to conventions
+    // 2. Fallback to conventions (covers all ApiRoutes keys + client-specific virtual routes)
     const routeMap: Record<ApiRouteType, string> = {
       data: '/api/v1/data',
       metadata: '/api/v1/meta',