fix(bug): BucketType.role in cooldowns could be based on a Thread rather than Role

[Enegg]

Summary

This PR fixes a subtle bug that went unnoticed due to a # type: ignore.
A Thread happens to structurally match abc.PrivateChannel (has .id and .me), which would cause an isinstance check to pass.

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, ...)

[shiftinv]

good catch! lgtm overall.
Regarding the underlying issue you mentioned - Thread really shouldn't match the PrivateChannel protocol in the first place, yeah. I don't think there are any unique attributes in DMChannel or GroupChannel, so this might have to go back to nominal subtyping? Doesn't necessarily have to be fixed in this PR, though.