Ensure numericality validations work with mutation

The detection of in-place changes caused a weird unexpected issue with
numericality validations. That validator (out of necessity) works on the
`_before_type_cast` version of the attribute, since on an `:integer`
type column, a non-numeric string would type cast to 0.

However, strings are mutable, and we changed strings to ensure that the
post type cast version of the attribute was a different instance than
the before type cast version (so the mutation detection can work
properly).

Even though strings are the only mutable type for which a numericality
validation makes sense, special casing strings would feel like a strange
change to make here. Instead, we can make the assumption that for all
mutable types, we should work on the post-type-cast version of the
attribute, since all cases which would return 0 for non-numeric strings
are immutable.

Fixes #17852

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