Updated _parse_due_date to handle accepting a JSON dictionary object

[jshiflet]

Whenever the HA integration tries pulling the reminders from the reminders-api, you could potentially see hundreds of error log entries like this:

2026-06-15 01:37:53.902 WARNING (MainThread) [custom_components.reminders_api.todo] Failed to parse due date: {'formatted': 'Jun 3, 2026 at 12:00\u202fPM CDT', 'iso': '2026-06-03T17:00:00Z', 'relative': 'last week', 'timezone': 'America/Chicago'}

This happens because the existing _parse_due_date function only accepted a string as value for dueDate. The reminders-api REST API returns a JSON dictionary object for the due date like this:

"dueDate" : {
      "formatted" : "Jul 1, 2026 at 4:00 PM CDT",
      "iso" : "2026-07-01T21:00:00Z",
      "relative" : "in 2 weeks",
      "timezone" : "America\/Chicago"
    },

This update allows _parse_due_date to also accept a JSON dictionary object, and will parse the iso string from the dueDate object to get the due date in a consistent format. If the dueDate value is already an ISO string, it will continue to parse it as before.