fix: resolve async stream resource leak on timeout

[joao-faria-dev]

Hello everyone! I got an error when running the tests and saw that there was already a issue related to this (#3686)

Summary

Fix async stream resource leak that caused ResourceWarning in Trio, basically replacing ByteStream.aiter() async generator with a iterator that can be properly closed via aclose()

Checklist

• [X] I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
• [X] 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.