[ty] Share class member source-method recovery (#28160)
## Summary
Move local source-method recovery from override checking into `Member`
in `list_members.rs`, so class-level checks can share it. We retain
functions represented by the member type, including property accessors,
and recover local definitions when a decorator replaces the function's
type.
Override checking still selects the same definitions and diagnostic
locations. This is a refactor with no behavior changes; deleted-method
handling remains unchanged.
