Form submissions with multiple values for the same key (e.g. ``<select
multiple>``) previously had every value but the last silently discarded,
because ``post_vars()`` called ``dict(parse_qsl(...))``. Switch the
implementation to return a ``MultiParams`` object built from
``parse_qs(...)``, mirroring the existing ``request.args`` shape so both
sides of the GET/POST surface behave consistently.
``MultiParams`` gains an ``items()`` method that yields (key, first_value)
pairs, matching ``__getitem__`` semantics, so it works in patterns such as
``dict(post_vars())`` and the existing ``_coerce_execute_write_payload``
dict comprehension.
Internal callers of ``_json_or_form_payload`` (``execute_write`` and
``stored_queries``) had ``isinstance(data, dict)`` guards intended to
validate JSON shape but also rejected the new ``MultiParams`` return on
the form path. Gate those guards on ``is_json`` so the form path passes
through unchanged.
The bundled ``my_plugin`` test fixture wraps ``post_vars()`` in ``dict()``
before passing to ``Response.json``, demonstrating the migration path for
plugins that need a JSON-serialisable mapping.
Closes#2425
