Summary

This change avoids warnings on Trio by explicitly closing all async generators rather than relying on the garbage collector to do so, as this may cause unpredictable behavior due to different GC implementations.

Checklist

• I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.