[`flake8-async`] Add fix safety section (`ASYNC116`) (#17497)

## Summary

This PR add the `fix safety` section for rule `ASYNC116` in
`long_sleep_not_forever.rs` for #15584

---------

Co-authored-by: dylwil3 <dylwil3@gmail.com>

crates/ruff_linter/src/rules/flake8_async/rules/long_sleep_not_forever.rs

@@ -34,6 +34,10 @@ use crate::rules::flake8_async::helpers::AsyncModule;
 /// async def func():
 ///     await trio.sleep_forever()
 /// ```
+///
+/// ## Fix safety
+///
+/// This fix is marked as unsafe as it changes program behavior.
 #[derive(ViolationMetadata)]
 pub(crate) struct LongSleepNotForever {
     module: AsyncModule,