add regression test

Author: alireza-shirzad

poly/src/evaluations/multivariate/multilinear/sparse.rs

@@ -450,6 +450,23 @@ mod tests {
         }
     }
 
+    #[test]
+    fn sparse_to_evaluations_matches_to_dense() {
+        let mut rng = test_rng();
+        const NV: usize = 8; // 2^8 = 256, small and fast
+
+        for _ in 0..25 {
+            // Make a sparse poly with ~sqrt(2^NV) non-zeros at random indices.
+            let sparse = SparseMultilinearExtension::<Fr>::rand(NV, &mut rng);
+            let dense_via_sparse = sparse.to_dense_multilinear_extension().evaluations;
+            let dense_via_to_evals = sparse.to_evaluations();
+            assert_eq!(
+                dense_via_to_evals, dense_via_sparse,
+                "to_evaluations must reproduce the dense vector exactly"
+            );
+        }
+    }
+
     #[test]
     fn evaluate_edge_cases() {
         // test constant polynomial