feat(schema): clarify error for types that cannot have a default value

The unknown/variant/geometry/geography branch reported "{type} fields must
default to null", which is ambiguous for the unknown type (reads as
"unrecognized fields") and awkwardly interpolates a parametrized type
(e.g. geometry(srid:...)) into a categorical sentence. Reword to
"type {} cannot have a default value" and align with the sibling
"Invalid {} value for {}: ..." messages.

Author: huan233usc

src/iceberg/schema_field.cc

@@ -142,7 +142,7 @@ Status ValidateDefault(const SchemaField& field, const Literal& value,
     case TypeId::kVariant:
     case TypeId::kGeometry:
     case TypeId::kGeography:
-      return InvalidSchema("Invalid {} value for {}: {} fields must default to null",
+      return InvalidSchema("Invalid {} value for {}: type {} cannot have a default value",
                            kind, field.name(), *field.type());
     default:
       break;

src/iceberg/test/schema_test.cc

@@ -242,7 +242,7 @@ TEST(SchemaTest, ValidateRejectsDefaultOnNonPrimitiveAndMustBeNullTypes) {
   auto status =
       geo_default.Validate(iceberg::TableMetadata::kSupportedTableFormatVersion);
   ASSERT_THAT(status, iceberg::IsError(iceberg::ErrorKind::kInvalidSchema));
-  EXPECT_THAT(status, iceberg::HasErrorMessage("must default to null"));
+  EXPECT_THAT(status, iceberg::HasErrorMessage("cannot have a default value"));
 }
 
 TEST(SchemaTest, ReassignIdsPreservesDefaultValues) {