fix(datetime.dst): pass Gregorian to tzinfo.dst()

[5j9]

Standard timezone libraries and databases expect standard Gregorian datetime objects to resolve DST offsets correctly. Passing a Jalali datetime object (self) causes two major issues:

1. It results in silent bugs because the database evaluates rules for the Jalali year (e.g., 1405 AD instead of the converted 2026 AD).
2. It can raise type/attribute errors if strict type-checking is enforced.

This converts the instance to its Gregorian representation using self.togregorian() before passing it to tzinfo.dst().

Also:

• Removes the outdated TODO comment on the dst method.
• Adds unit tests using zoneinfo to verify DST calculations for both aware and naive jdatetime instances.