fix: Merge the latest code from the main branch

Author: gty404

src/iceberg/json_internal.cc

@@ -401,7 +401,7 @@ Result<std::unique_ptr<PartitionField>> PartitionFieldFromJson(
   ICEBERG_ASSIGN_OR_RAISE(auto field_id, GetJsonValue<int32_t>(json, kFieldId));
   ICEBERG_ASSIGN_OR_RAISE(
       auto transform,
-      GetJsonValue<std::string>(json, kTransform).and_then(TransformFunctionFromString));
+      GetJsonValue<std::string>(json, kTransform).and_then(TransformFromString));
   ICEBERG_ASSIGN_OR_RAISE(auto name, GetJsonValue<std::string>(json, kName));
   return std::make_unique<PartitionField>(source_id, field_id, name,
                                           std::move(transform));

test/json_internal_test.cc

@@ -102,7 +102,7 @@ TEST(JsonInternalTest, SortOrder) {
 }
 
 TEST(JsonInternalTest, PartitionField) {
-  auto identity_transform = std::make_shared<IdentityTransformFunction>();
+  auto identity_transform = Transform::Identity();
   PartitionField field(3, 101, "region", identity_transform);
   nlohmann::json expected_json =
       R"({"source-id":3,"field-id":101,"transform":"identity","name":"region"})"_json;
@@ -125,7 +125,7 @@ TEST(JsonPartitionTest, PartitionSpec) {
                SchemaField(3, "region", std::make_shared<StringType>(), false),
                SchemaField(5, "ts", std::make_shared<LongType>(), false)});
 
-  auto identity_transform = std::make_shared<IdentityTransformFunction>();
+  auto identity_transform = Transform::Identity();
   PartitionSpec spec(schema, 1,
                      {PartitionField(3, 101, "region", identity_transform),
                       PartitionField(5, 102, "ts", identity_transform)});