Cover the input-validation contract dispatch_run relies on:
- no declared schema → inputs pass through unchanged (regression site
that previously stripped runtime keys like fired_at / last_fired_at
and broke Jinja templates).
- declared schema, valid inputs → passthrough validated.
- declared schema, invalid inputs → DispatchError (uniform exception
type, not raw jsonschema.ValidationError).
Plus the DispatchError exception identity (Exception subclass, message
preserved, isinstance-friendly for the dispatch layer's consumers).
4 tests, pure unit.
