fix: clean up filter merging logic and add combined filter+query test

- Replace incorrect Object.keys-on-array guard with direct filter
  conversion, matching the pattern used in getEntityFacetsByPredicate
- Add backend router test covering combined filter query param + body
  query predicate being forwarded independently to entitiesBatch

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Signed-off-by: Fredrik Adelöw <freben@gmail.com>

packages/catalog-client/src/CatalogClient.ts

@@ -249,11 +249,8 @@ export class CatalogClient implements CatalogApi {
       }
       filterPredicate = query;
       if (filter !== undefined) {
-        const filterValue = this.getFilterValue(filter);
-        if (filterValue && Object.keys(filterValue).length > 0) {
-          const converted = convertFilterToPredicate(filter);
-          filterPredicate = { $all: [filterPredicate, converted] };
-        }
+        const converted = convertFilterToPredicate(filter);
+        filterPredicate = { $all: [filterPredicate, converted] };
       }
     }