Add require_one: fetch exactly one row, error if none (#2974) (#3164)

Adds a non-optional counterpart to one(): where one() returns
Option<Item>, require_one() returns Item directly and yields
DbErr::RecordNotFound when no row matches, so call sites can use ? instead
of unwrapping/matching an Option. This mirrors the fetch_one vs
fetch_optional distinction common in other query APIs.

Implemented on the base Selector and SelectorRaw executors, with ergonomic
wrappers on Select, SelectTwo, and SelectTwoRequired. Mirrored to
sea-orm-sync via make-sync.sh.