Status length validation spec updates

[mjankowski]

This is a slightly different approach to the spec portion of #30091

• Pull out a helper method to DRY up what was repeated setup/double calls for the statuses
• Change example titles to reference character limit, not specific amount
• Add explicit const stub for the max_chars value, so that the spec remains relevant to that value even if the version in the validator were changed or made dependent on env var

I think in theory that other could be rebased against this and remove all it's spec changes and be reviewed just on the merits of the configurability.

Separately, I noticed while in here we sort of have a mix of styles in validator specs, with some doing this stub out of the activemodel errors object, and some doing a (what I think is preferred) more native usage of the validator via an in-spec class. Might do a round of updating there to align them all style wise to that latter approach.