test: add lexer unit tests for parser/lexer.cpp

- Add 54 unit tests covering Lexer class:
  - Basic tokenization (SELECT, identifiers, numbers, strings)
  - Keywords case insensitivity
  - Operators (=, <>, <, <=, >, >=, +, -, *, /)
  - Delimiters (parentheses, comma, semicolon, dot)
  - Whitespace and newline handling
  - SQL comments (-- style)
  - Position tracking (line/column)
  - peek_token() behavior
  - Error handling (unterminated strings, invalid chars)
  - Complex SQL statements (CREATE TABLE, INSERT, UPDATE, DELETE, JOIN)

Author: poyrazK

CMakeLists.txt

@@ -136,6 +136,7 @@ if(BUILD_TESTS)
     add_cloudsql_test(raft_protocol_tests tests/raft_protocol_tests.cpp)
     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(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)