Validate only the combined budget, not either field's own limit.
Split out because the two kinds of overshoot need opposite handling in the
criteria agent's structured-output loop: a per-field overshoot is a model
mistake against a limit the schema publishes, so it is worth retrying, while
the combined budget is not in the schema at all and half of it is the user's
own objective. See _raise_terminal_goal_state_size_error.
