test: add query_executor unit tests

Add comprehensive unit tests for QueryExecutor covering:
- Constructor and setup methods
- CREATE/DROP TABLE with various types
- INSERT single/multiple rows with column lists
- SELECT with projections, WHERE, ORDER BY, LIMIT/OFFSET
- Aggregates: COUNT, SUM, GROUP BY
- UPDATE and DELETE with conditions
- Transaction control: BEGIN, COMMIT, ROLLBACK
- Transaction isolation (MVCC)
- CREATE/DROP INDEX
- JOIN operations (inner, left)
- Invalid SQL handling

Part of ongoing coverage improvement effort.

Author: poyrazK

CMakeLists.txt

@@ -137,6 +137,7 @@ if(BUILD_TESTS)
     add_cloudsql_test(storage_manager_tests tests/storage_manager_tests.cpp)
     add_cloudsql_test(rpc_server_tests tests/rpc_server_tests.cpp)
     add_cloudsql_test(operator_tests tests/operator_tests.cpp)
+    add_cloudsql_test(query_executor_tests tests/query_executor_tests.cpp)
 
     add_custom_target(run-tests
         COMMAND ${CMAKE_CTEST_COMMAND}