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
  • [x] I have formatted the code properly by running pdm lint
  • [ ] I have type-checked the code by running pdm pyright
• [x] 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, ...)