test: cover forbidden nested type promotions

Assert that promotion helpers reject nested type targets for unknown and regular primitive source types.

Co-authored-by: Codex <codex@openai.com>

Author: manuzhang

src/iceberg/test/type_test.cc

@@ -258,6 +258,15 @@ INSTANTIATE_TEST_SUITE_P(Primitive, TypeTest, ::testing::ValuesIn(kPrimitiveType
 INSTANTIATE_TEST_SUITE_P(Nested, TypeTest, ::testing::ValuesIn(kNestedTypes),
                          TypeTestCaseToString);
 
+TEST(TypeTest, PromotionToNestedTypesIsForbidden) {
+  for (const auto& nested_type : kNestedTypes) {
+    SCOPED_TRACE(nested_type.name);
+
+    EXPECT_FALSE(iceberg::IsPromotionAllowed(iceberg::unknown(), nested_type.type));
+    EXPECT_FALSE(iceberg::IsPromotionAllowed(iceberg::int32(), nested_type.type));
+  }
+}
+
 TEST(TypeTest, Equality) {
   std::vector<std::shared_ptr<iceberg::Type>> alltypes;
   for (const auto& test_case : kPrimitiveTypes) {