Previously we had primarily tested the behavior of these attributes by
calling `.new`, allowing this to slip through the cracks. There were a
few ways in which they were behaving incorrectly.

The biggest issue was that attempting to read the attribute would
through a `MissingAttribute` error. We've corrected this by returning
the default value when the attribute isn't backed by a database column.
This is super special cased, but I don't see a way to avoid this
conditional. I had considered handling this higher up in
`define_default_attribute`, but we don't have the relevant information
there as users can provide new defaults for database columns as well.

Once I corrected this, I had noticed that the attributes were always
being marked as changed. This is because the behavior of
`define_default_attribute` was treating them as assigned from
`Attribute::Null`.

Finally, with our new implementation, `LazyAttributeHash` could no
longer be marshalled, as it holds onto a proc. This has been corrected
as well. I've not handled YAML in that class, as we do additional work
higher up to avoid YAML dumping it at all.

Fixes #25787
Close #25841