eca_form_field_set_weight.weight is typed string while the render base uses eca_integer_or_token

Noticed while working #3590380 (closed).

action.configuration.eca_form_field_set_weight.weight is typed string, while the form element is a #type textfield with #eca_token_replacement — structurally the identical shape to eca_render.action_base's weight.

It was never affected by the token-destruction bug in #3590380 (closed), precisely because string does no casting. But after that fix ECA carries two different types for one concept: eca_integer_or_token in the render base and string here.

The issue #3590380 (closed) asked for the resolution to be applied consistently across ECA, so this is the remaining inconsistency.

Worth noting that simply retyping this key to eca_integer_or_token is the direction that keeps ECA consistent, but the reverse — retyping the render base to string — would convert stored integers to strings on next save, so it should not be done that way.