Add parser unit tests (81 tests) and fix IF NOT EXISTS parsing

- Created tests/parser_tests.cpp with comprehensive coverage of:
  - SELECT with all clauses (WHERE, GROUP BY, HAVING, ORDER BY, LIMIT, OFFSET, JOINs)
  - CREATE TABLE/INDEX, INSERT, UPDATE, DELETE, DROP
  - Transaction statements (BEGIN, COMMIT, ROLLBACK)
  - Expression parsing (binary, unary, IN, IS NULL, functions)
  - Error handling and edge cases

- Fixed parse_create_table() to handle IF NOT EXISTS in correct order
  (was checking for NOT before IF, now correctly checks IF before NOT)

- Note: PRIMARY KEY not supported by lexer (missing PRIMARY/KEY keywords)

Author: poyrazK

CMakeLists.txt

@@ -137,6 +137,7 @@ if(BUILD_TESTS)
     add_cloudsql_test(columnar_table_tests tests/columnar_table_tests.cpp)
     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(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)

src/parser/parser.cpp

@@ -287,7 +287,10 @@ std::unique_ptr<Statement> Parser::parse_create_table() {
     }
 
     /* IF NOT EXISTS */
-    if (consume(TokenType::Not)) {
+    if (consume(TokenType::If)) {
+        if (!consume(TokenType::Not)) {
+            return nullptr;
+        }
         if (!consume(TokenType::Exists)) {
             return nullptr;
         }