feat(firestore): return listBackupsResponse from firestore:backups:list --json

The `firestore:backups:list --json` command now returns the full `listBackupsResponse` object instead of just the `backups` array. This provides more information to the user, including a list of unreachable locations.

Author: google-labs-jules[bot]

src/commands/firestore-backups-list.ts

@@ -30,7 +30,5 @@ export const command = new Command("firestore:backups:list")
       );
     }
 
-    // TODO: Consider returning listBackupResponse instead for --json. This will
-    // be a breaking change but exposes .unreachable, not just .backups.
-    return backups;
+    return listBackupsResponse;
   });