Always ``list``-cast the ``mngrs`` input to
``.trionics.gather_contexts()`` and ensure its size otherwise raise
a ``ValueError``.

Turns out that trying to pass an inline-style generator comprehension
doesn't seem to work inside the ``async with`` expression? Further, in
such a case we can get a hang waiting on the all-entered event
completion when the internal mngrs iteration is a noop. Instead we
always greedily check a size and error on empty input; the lazy
iteration of a generator input is not beneficial anyway since we're
entering all manager instances in concurrent tasks.
