[ty] Preserve property subclass types and accessors (#27833)
## Summary
Subclasses of `property` lost their getter type during construction, and
applying an inherited `.setter` reduced subsequent attribute access to
`Unknown`. Preserve the descriptor's nominal class and accessor types
when it inherits the standard property implementation.
Generalize the existing `enum.property` handling to ordinary property
subclasses, retain generic specializations through accessor replacement,
and recognize inherited descriptor methods without overriding custom
behavior. Subclasses with custom constructors, descriptor methods, or
accessor attributes continue through ordinary inference.
Closes https://github.com/astral-sh/ty/issues/4302
## Test plan
Added mdtests for getter and setter inference, invalid setter
assignments, direct construction, all three accessor-copy methods,
generic and `enum.property` subclasses, custom accessor methods and
attributes, custom descriptor behavior and constructors, and subclass
truthiness. A variance regression verifies that mutable state on a
generic property subclass makes its owning class invariant and prevents
unsafe writes through a widened owner. Existing incrementality tests
also cover the method-recognition path.
