[ty] Preserve nominal type of enum.property instances (#25849)
## Summary
This is a follow-up to #25681 that preserves the nominal class of
synthesized property instances created by `enum.property`.
Previously, `PropertyInstanceType` always fell back to
`builtins.property`. That discarded the `enum.property` identity, so
assignments to `enum.property` failed and enum-specific members such as
`name`, `clsname`, and `member` were unavailable. The originating known
class is now retained through accessor replacement, type
transformations, member lookup, descriptor dispatch, relations, `super`,
and display.
The regression coverage also checks precise getter and setter behavior.
A TODO documents the remaining limitation that constructing a subclass
of `enum.property` currently collapses the result to `enum.property`.
## Test Plan
Added/updated mdtests.
