style: Fix RuboCop offenses in TableFunction

- Add empty lines before assertions (auto-correctable)
- Disable EmptyClass warning (acceptable for C extension placeholder)
- Refactor long test method into helper methods
- Rename short parameter names to meet 3-char minimum

All 6 tests still passing.

Author: suketa

lib/duckdb/table_function.rb

@@ -25,7 +25,10 @@ module DuckDB
   #     conn.register_table_function(tf)
   #   end
   #
+  # rubocop:disable Lint/EmptyClass
   class TableFunction
     # TableFunction.create is defined in C extension
+    # Additional Ruby methods can be added here in future phases
   end
+  # rubocop:enable Lint/EmptyClass
 end

test/duckdb_test/table_function_test.rb

@@ -7,6 +7,7 @@ class TableFunctionTest < Minitest::Test
     # Test 1: Create and destroy
     def test_create_and_destroy
       tf = DuckDB::TableFunction.create
+
       assert_instance_of DuckDB::TableFunction, tf
       tf.destroy
     end
@@ -47,27 +48,38 @@ def test_create_with_block
         tf.name = 'my_function'
         :block_result
       end
+
       assert_equal :block_result, result
     end
 
     # Test 7: Register without callbacks (should fail gracefully)
     # TODO: Enable this test once database connection issue is resolved
     def _test_register_without_callbacks
-      db = DuckDB::Database.new
-      conn = db.connect
-
-      tf = DuckDB::TableFunction.create
-      tf.name = 'incomplete_function'
-      tf.add_parameter(DuckDB::LogicalType::BIGINT)
+      database, conn, table_function = setup_incomplete_function
 
       # Should fail because no bind/init/execute callbacks set
       assert_raises(DuckDB::Error) do
-        conn.register_table_function(tf)
+        conn.register_table_function(table_function)
       end
 
-      tf.destroy
+      cleanup_function(table_function, conn, database)
+    end
+
+    private
+
+    def setup_incomplete_function
+      database = DuckDB::Database.new
+      conn = database.connect
+      table_function = DuckDB::TableFunction.create
+      table_function.name = 'incomplete_function'
+      table_function.add_parameter(DuckDB::LogicalType::BIGINT)
+      [database, conn, table_function]
+    end
+
+    def cleanup_function(table_function, conn, database)
+      table_function.destroy
       conn.disconnect
-      db.close
+      database.close
     end
   end
 end