Add real async iterator for ByteStreams

[akx]

Summary

Previously, __aiter__ was implemented as a generator function, and as such could get garbage collected when not completely exhausted in a way that has (recent versions of) trio complaining:

$ uv pip list | grep io
[... snip ...]
Package                    Version
anyio                      4.12.1
trio                       0.31.0
[... snip ...]
$ uv run pytest tests/ -k test_write_timeout[trio]
[... snip ...]
E ResourceWarning: Async generator 'httpx._content.ByteStream.__aiter__' was garbage collected before it had been exhausted. Surround its use in 'async with aclosing(...):' to ensure that it gets cleaned up as soon as you're done using it.

The above exception was the direct cause of the following exception:

E pytest.PytestUnraisableExceptionWarning: Exception ignored in: <async_generator object ByteStream.__aiter__ at 0x1075593c0>
[... snip ...]

Using a real, if trivial, object to do this causes less complaints.

Fixes #3686
Closes #3700 (fix for the same issue)

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.
  • Retained pristine test coverage to check that the iterator does indeed adhere to the iterator protocol by returning itself when iterated upon.
• I've updated the documentation accordingly.
  • No documentation changes required.