fallback: pass object_pairs_hook a list of pairs - #744

The pure-Python Unpacker passes object_pairs_hook a generator, while the C extension and the docstring give it a list of pairs. On PyPy or with MSGPACK_PUREPYTHON=1:

• a hook that reads the pairs twice (for example to reject duplicate keys) returns {};
• pairs[0] and len(pairs) raise TypeError;
• a hook that ignores its argument leaves the map body unread, so the following objects are decoded from inside the map (ExtraData from unpackb).

This builds the list eagerly, keeping the strict_map_key check on each key; it is Finding 5 of #683. The check now runs whether or not the hook iterates, which matches the fallback's dict path and a hook like list. pytest test and MSGPACK_PUREPYTHON=1 pytest test pass; the new test fails on main under the fallback only.

Written with AI assistance (Claude); I have reviewed the change.