(py) Mid-stream body error raises StopAsyncIteration, so truncated streams look like clean EOF

[barjin]

🤖 Found by Claude ultrareview — automated high-effort code review. Please verify independently before acting.

Location: impit-python/src/response.rs:158 (PyResponseAsyncBytesIterator::__anext__)

A mid-stream body error is mapped to PyStopAsyncIteration:

Some(Err(e)) => {
    if let Some(parent) = parent_response {
        Python::attach(|py| {
            if let Ok(mut parent_ref) = parent.try_borrow_mut(py) {
                parent_ref.inner_state = InnerResponseState::StreamingClosed;
                parent_ref.is_closed = true;
            }
        });
    }
    Err(pyo3::exceptions::PyStopAsyncIteration::new_err(format!(...)))
}

Impact: Raising StopAsyncIteration signals normal end-of-iteration to async for. So a connection reset or truncated chunked transfer mid-body silently ends the loop as if the stream completed, and the caller processes a partial body believing it is complete — a silent data-integrity bug. The error message is formatted into the exception but async for never surfaces it.

Additionally, unlike the clean-EOF branch, this path leaves is_stream_consumed = false, so a subsequent read()/content/text reports StreamClosed instead of the real failure.

Suggested direction: propagate stream errors as a real exception type (the classified ImpitError), not StopAsyncIteration, and set the consumed/closed flags consistently with the EOF branch.