Add direct unit tests for SQL validation helpers

[todo-mdx]

TESTING: helpers/sql-validation.ts is only tested indirectly via CRUD tests.

Location: src/helpers/sql-validation.ts

TDD Steps:

• RED: Write direct unit tests for: isValidColumnName() with valid/invalid patterns, isAllowedColumn() with whitelist, validateColumns() batch validation
• GREEN: All tests should pass
• REFACTOR: Test SQL injection attack patterns