fallback: match the C extension's strict_map_key error message - #754

Summary

The pure-Python fallback and the C extension raise different error messages when a
map key has a type that strict_map_key disallows.

The C extension raises:

ValueError: int is not allowed for map key when strict_map_key=True

The pure-Python fallback raises:

ValueError: <class 'int'> is not allowed for map key

So the message a caller sees depends on whether the C extension is available, which
makes it unreliable to log, match, or assert on. This aligns the fallback with the C
extension: it now uses type(key).__name__ instead of str(type(key)) and includes
the same when strict_map_key=True suffix, so both backends report the identical error.

Changes

• msgpack/fallback.py: use the same message as unpack_callback_map_item in
  unpack.h, in both the dict and object_pairs_hook map paths.
• test/test_except.py: add test_strict_map_key_error_message, which asserts the
  message and runs under both the C extension and the fallback.

Notes

No behavior change beyond the message text. The full suite passes under both backends
(MSGPACK_PUREPYTHON=1 and the compiled extension).