fix(validator): prevent type diff bug in form data parsing (#4753)

* fix(validator): prevent prototype pollution in form data parsing

Use Object.create(null) instead of {} for the form data object and
replace `key in form` with `Object.hasOwn(form, key)` to prevent
__proto__ keys in FormData from polluting the object prototype chain.

Add regression tests verifying null prototype and safe __proto__ handling.

* chore: clean up comments in validator.test.ts

Remove commented-out lines in the validator test.