update_job() assumed the schedule value was always a pre-parsed dict
and called .get() on it directly. When the API passes a raw string
like "every 10m", this crashed with AttributeError.
The create path already handles this correctly by calling
parse_schedule() on the incoming string. The fix adds the same
normalization to the update path: if the schedule is a string,
parse it into a dict before proceeding.
Closes#10129
c4674cbe21