feat(channel): add GroupChannel.get_partial_message

[shiftinv]

Summary

Now that Message.channel can potentially be a GroupChannel (in user app interactions, see #1233), having get_partial_message available on all but one of the possible types is annoying.
This fixes that, adding get_partial_message to the common interface that all messageable channel types support, even if partial messages in this context may not be incredibly useful.

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, ...)