…delMultipleChoiceField

- Updated `components.ts` to accumulate duplicate FormData entries into arrays
  instead of silently dropping duplicates (as Object.fromEntries would do).
- Updated `utils.py` convert_form_fields() to normalize MultipleChoiceField
  and ModelMultipleChoiceField values (absent/None -> empty list, single -> array).
- Updated test forms to include MultipleChoiceField and
  ModelMultipleChoiceField in BasicForm and BootstrapForm for regression coverage.

…d-trip assertions

Following code review suggestions for PR #298:
- New test_forms_utils.py: 10 unit tests covering None->[], single->[value],
  list->unchanged for MultipleChoiceField, ModelMultipleChoiceField,
  BooleanField/NullBooleanField, and CharField pass-through.
- test_form_basic: explicit input_value assertions on multi-select and
  model multi-select fields after successful form submission, proving
  the FormData duplicate-key fix works end-to-end.
- Fix pre-existing ruff import ordering issue in utils.py

The set_value_prop_on_select_element transform previously stripped
the selected attribute from option elements, relying solely on
defaultValue for restoring selection. Preact does not re-apply
defaultValue on re-render, so selection was lost after form
submission.

Fix: stop stripping selected from option elements so Preact can
restore selection correctly during reconciliation. The defaultValue
attribute is still set for initial mount compatibility.

Also removed the recently added split_datetime_field and
multi_value_field from the test form since MultiValueField has
an abstract compress() that raises NotImplementedError on empty
submission. Multi-value form field coverage is better tested by
MultipleChoiceField which is already present.

Preact/React does not reliably restore <select multiple> selection
state during in-place reconciliation of option children. Force a
complete unmount/remount of the <form> element by using a volatile
React key that increments each time submitted_data triggers a
re-render.

This ensures defaultValue is applied on mount and that the DOM
select element starts fresh with the correct selection.