Namely renaming and massively simplifying it to a new
`test_ext_types_over_ipc` which avoids all the wacky "parent dictates
what sender should be able to send beforehand"..
Instead keep it simple and just always try to send the same small set of
types over the wire with expect-logic to handle each case,
- use the new `dec_hook`/`ext_types` args to `mk_[co]dec()` routines for
pld-spec ipc transport.
- always try to stream a small set of types from the child with logic to
handle the cases expected to error.
Other,
- draft a `test_pld_limiting_usage` to check runtime raising of bad API
usage; haven't run it yet tho.
- move `test_custom_extension_types` to top of mod so that the
`enc/dec_nsp()` hooks can be reffed from test parametrizations.
- comment out (and maybe remove) the old routines for
`iter_maybe_sends`, `test_limit_msgspec`, `chk_pld_type`.
XXX TODO, turns out the 2 failing cases from this suite have exposed an
an actual bug with `MsgTypeError` unpacking where the `ipc_msg=` input
is being set to `None` ?? -> see the comment at the bottom of
`._exceptions._mk_recv_mte()` which seems to describe the likely
culprit?
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