Add expression unit tests (62 tests)

Created tests/expression_tests.cpp covering:
- BinaryExpr: arithmetic (+,-,*,/), comparisons (=,<>,<,<=,>,>=), logical (AND,OR)
- UnaryExpr: negation (-), NOT
- ColumnExpr: simple, qualified, not found, null tuple
- ConstantExpr: int64, float64, text, bool, null
- ParameterExpr: binding, null params
- FunctionExpr: basic, distinct, multiple args
- InExpr: IN, NOT IN
- IsNullExpr: IS NULL, IS NOT NULL
- Clone tests for all expression types
- to_string tests for all expression types
- Complex expressions (arithmetic and logical)
- Expression type queries

Note: Division (/) always returns float64 per expression.cpp implementation

Author: poyrazK

CMakeLists.txt

@@ -140,6 +140,7 @@ if(BUILD_TESTS)
     add_cloudsql_test(heap_table_tests tests/heap_table_tests.cpp)
     add_cloudsql_test(lexer_tests tests/lexer_tests.cpp)
     add_cloudsql_test(parser_tests tests/parser_tests.cpp)
+    add_cloudsql_test(expression_tests tests/expression_tests.cpp)
     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)