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Give the list-pair helpers a header of their own (#1534)

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## What do these changes do?

`_list_getitem_ref()` and `_list_item_gone()` were left behind in
`hashtable.h` by #1533 without being about the hash table, and with no
caller there any more. They move into a new
`multidict/_multilib/pair.h`, along with a new `unpack_pair()` that both
of their users now share: `itemsview.__contains__()` in `views.h` and
`_md_parse_item()` in `bulk_update.h` each kept their own copy of the
exact-tuple and exact-list fast paths, including the free-threaded
reference dance the list branch needs.

Each caller keeps its own slow path, because the two disagree on whether
a wrong length or a foreign type is an error or a plain "no";
`unpack_pair()` reports those back rather than acting on them. Folding
the wrong-length case into the foreign-type one, which would drop a
state
from the result, is wrong: it lets an exact list reach the general path,
which re-reads the length and then indexes without the atomic accessor.
`test_items_contains_list_shrunk_by_another_thread` catches it.

C extension only. There is no pure-Python counterpart to keep in sync,
since the split is a C header layout question; `_multidict_py.py` is
untouched.

## Are there changes in behavior for the user?

No.

## Is it a substantial burden for the maintainers to support this?

No, it removes a duplicated fast path and finishes the split #1533
started.

## Related issue number

None; this is the follow-up #1533 pointed at when it left these two
helpers behind in `hashtable.h`.

## Checklist

- [x] I think the code is well written
- [x] Unit tests for the changes exist — N/A, no new behaviour; the
      existing suite covers both call sites, including the free-threaded
      `test_items_contains_list_shrunk_by_another_thread`
- [x] Documentation reflects the changes — `AGENTS.md` header table
- [ ] If you provide code modification, please add yourself to
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