test(drive): assert the sibling-bound count's derived post

The descending-page test bound a like count to a limited sibling to
show the sibling's limit applies before values derive, but only
compared the materialized and verified results with each other. It
now asserts the count covers exactly the sibling's single post, with
the like count that post carries.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

Author: QuantumExplorer

packages/rs-drive/src/drive/contract/insert/insert_contract/v0/tests/composite_query_e2e_tests.rs

@@ -707,6 +707,16 @@ fn should_preserve_descending_documents_and_key_ordered_counts() {
         .expect("verifies");
     assert_eq!(verified.page_documents, materialized.page_documents);
     assert_eq!(verified.sub_results, materialized.sub_results);
+    // The count bound to the sibling derived exactly the one post the
+    // sibling's limit left it: A carries two likes, B one.
+    let sibling_posts = post_ids_of(&verified.sub_results[2], "postId");
+    assert_eq!(sibling_posts.len(), 1);
+    let expected_likes = if sibling_posts[0] == POST_A { 2 } else { 1 };
+    assert_eq!(
+        counts(&verified.sub_results[3]),
+        BTreeMap::from([(sibling_posts[0], expected_likes)]),
+        "the sibling-bound count covers the sibling's single derived post"
+    );
 }
 
 /// The full round trip: the server's materialized result and the