Handle locked campsites in UseDirect/ReserveCalifornia provider

[aniketde]

Summary

• Treat locked campsites as available: When reservations are cancelled on ReserveCalifornia, sites enter a timed lock (typically until 8 AM the next day). Previously these were reported as "Unavailable" because only IsFree was checked. Now, sites with a future Lock timestamp are correctly treated as "Available".
• Proper typing for Lock field: Changed Lock: Any to Lock: Optional[datetime.datetime] in the UseDirectAvailabilitySlice model.
• Informational logging: When locked sites are detected, a log message tells users how many locked sites were found and that they will become bookable at their scheduled unlock time.

Fixes #329
Fixes #298

Test plan

• Verified with live API: Crystal Cove Beach Cottages (campground 757) — previously returned 0 available sites, now correctly returns 15 locked sites as available
• Verified all edge cases: IsFree=True → Available, Lock=future → Available, Lock=past → Unavailable, Lock=None → Unavailable
• Verified Pydantic model parses Lock field correctly for null, datetime strings, and datetime objects
• CLI end-to-end test confirms sites show up with booking links