fix: replay index specs verbatim instead of reconstructing them

backup/dump.go

@@ -147,11 +147,25 @@ func dumpIndexes(ctx context.Context, w *Writer, db *mongo.Database, name string
 	}
 	defer cur.Close(ctx)
 
-	var specs []bson.M
+	// The specs are read as raw BSON and re-encoded as extended JSON, one
+	// element per index, so key order and every option the server reported —
+	// partialFilterExpression, collation, weights and the rest — survive
+	// verbatim. Decoding into bson.M would lose compound key order, and
+	// reconstructing an index from a hand-picked set of options would drop
+	// whatever was not picked.
+	var specs []bson.Raw
 	if err := cur.All(ctx, &specs); err != nil {
 		return fmt.Errorf("read indexes on %s: %w", name, err)
 	}
-	raw, err := json.Marshal(specs)
+	encoded := make([]json.RawMessage, 0, len(specs))
+	for _, spec := range specs {
+		ej, err := bson.MarshalExtJSON(spec, false, false)
+		if err != nil {
+			return fmt.Errorf("encode indexes on %s: %w", name, err)
+		}
+		encoded = append(encoded, ej)
+	}
+	raw, err := json.Marshal(encoded)
 	if err != nil {
 		return fmt.Errorf("encode indexes on %s: %w", name, err)
 	}