fix(channel): improve handling of placeholder guilds in GuildChannel and Thread

[shiftinv]

Summary

This avoids errors related to Object guilds on guild-dependent channel properties when used with user apps. Stubbing methods isn't ideal, but it's arguably better than throwing AttributeErrors at random.

Closely related to #1236, the future of which is still very much undecided at this point.

(context: since interactions don't receive a (proper) guild object, we generally substitute it for Object if necessary. This works fine:tm: for the most part, but some properties (rightfully) expect an actual Guild object)

Checklist

• If code changes were made, then they have been tested
  • I have updated the documentation to reflect the changes
  • I have formatted the code properly by running pdm lint
  • I have type-checked the code by running pdm pyright
• This PR fixes an issue
• This PR adds something new (e.g. new method or parameters)
• This PR is a breaking change (e.g. methods or parameters removed/renamed)
• This PR is not a code change (e.g. documentation, README, ...)