test: round-trip fixed-size list scalars through protobuf

Covers the ConstantArray metadata path for fixed-size list dtypes
(e.g. a UUID column stored as fixed_size_list(u8)[16]): serialization
writes the scalar as a protobuf ListValue, and deserialization
previously rejected it with "expected List dtype for ListValue".

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Signed-off-by: Frederic Branczyk <fbranczyk@gmail.com>

Author: brancz

vortex-array/src/scalar/tests/round_trip.rs

@@ -254,6 +254,25 @@ mod tests {
         assert_eq!(extracted.as_slice(), &large_binary);
     }
 
+    // Test that fixed-size list scalars round-trip through protobuf. This is
+    // the path taken by a ConstantArray of a fixed-size list dtype (e.g. a
+    // UUID column stored as fixed_size_list(u8)[16]), whose scalar metadata
+    // is serialized as a protobuf ListValue.
+    #[test]
+    fn test_protobuf_fixed_size_list_round_trip() {
+        let fsl = Scalar::fixed_size_list(
+            Arc::new(DType::Primitive(PType::U8, Nullability::NonNullable)),
+            (0u8..16)
+                .map(|i| Scalar::primitive(i, Nullability::NonNullable))
+                .collect(),
+            Nullability::NonNullable,
+        );
+
+        let pb_fsl = pb::Scalar::from(&fsl);
+        let round_tripped = Scalar::from_proto(&pb_fsl, &SESSION).unwrap();
+        assert_eq!(fsl, round_tripped);
+    }
+
     // Test that nullable and non-nullable types are preserved
     #[test]
     fn test_nullability_preservation() {