`packb` accepts a dict with a tuple key but the result cannot be `unpackb`'d (round-trip asymmetry)

[BoxiYu]

packb serializes a mapping with a tuple key without error, but the produced bytes cannot be deserialized: unpackb decodes the tuple key as a list, then fails to use it as a dict key.

import msgpack
b = msgpack.packb({(1, 2): 0})            # succeeds (5 bytes)
msgpack.unpackb(b, strict_map_key=False)  # TypeError: unhashable type: 'list'

Environment: msgpack-python 1.2.1, Python 3.13.12.

Expected: either packb should raise on a key that cannot survive a round trip, or the limitation should be documented. Currently it is silent at pack time and only fails on unpack. Found by round-trip testing (106/3000 random structured inputs, all reducing to a tuple-keyed dict).

[methane]

msgpack is a JSON-like cross-language format, unlike pickle which is a language-specific format. Therefore, it should be expected from the beginning that multiple Python object types may map to the same msgpack type, and it may not be possible to restore them to their original form when unpacking.
Just as lists/tuples cannot be round-tripped in JSON, it is natural that they cannot be round-tripped in msgpack either.

To avoid TypeError, you can specify use_list=False in addition to strict_map_key=False. However, this is not the usage I recommend. Basically, only strings should be used as keys in maps. Therefore, I do not intend to add such usage notes for ordinary users. Human cognitive resources are limited, input tokens for AI have costs, and only truly useful information should be included in the documentation.

I will think a little more about whether to add the strict_map_key option to the packer or just document it.