fix(tvix/castore/import): assert end of stream

Once we break out with the root node, there may be no more elements in
the stream.

Change-Id: I6f5fc5662095aa2b2a56bcad506d25520d9ad00c
Reviewed-on: https://cl.tvl.fyi/c/depot/+/11592
Reviewed-by: Connor Brewster <cbrewster@hey.com>
Tested-by: BuildkiteCI
Autosubmit: flokli <flokli@flokli.de>

tvix/castore/src/import/mod.rs

@@ -134,6 +134,11 @@ where
         }
     };
 
+    assert!(
+        entries.count().await == 0,
+        "Tvix bug: left over elements in the stream"
+    );
+
     assert!(
         directories.is_empty(),
         "Tvix bug: left over directories after processing ingestion stream"