behave more like dataclasses when creating them

behave more like dataclasses when creating them

A change in the mechanics of how Python dataclasses are applied to classes
that use :class:`.MappedAsDataclass` or
:meth:`.registry.mapped_as_dataclass`, to no longer modify the
`__annotations__` collection that's on the class, instead manipulating
regular class-bound attributes in order to satisfy the class requirements
for the dataclass creation function.  This works around an issue that has
appeared in Python 3.14.1, provides for a much simpler implementation, and
also maintains accurate typing information about the attributes as the
dataclass is built.

Fixes: #13021
Change-Id: I6ae13db7f647ad04e202667d69f2b1bb385c032d