Follow-up to Magaav's safe sync policy. Two gaps in the canonicalizer
caused false diffs or silent drift:
1. discord.py's AppCommand.to_dict() omits nsfw, dm_permission, and
default_member_permissions — those live only on attributes. The
canonicalizer was reading them via payload.get() and getting defaults
(False/True/None), while the desired side from Command.to_dict(tree)
had the real values. Any command using non-default permissions
false-diffed on every startup. Pull them from the AppCommand
attributes via _existing_command_to_payload().
2. contexts and integration_types weren't canonicalized at all, so
drift in either was silently ignored. Added both to
_canonicalize_app_command_payload (sorted for stable compare).
Also normalized default_member_permissions to str-or-None since the
server emits strings but discord.py stores ints locally.
Added regression tests for both gaps.
b61ac8964b