(py) Streaming read()/aread() hardcode NetworkError, discarding the real timeout/protocol/decode cause

[barjin]

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

Location: impit-python/src/response.rs:401 (aread) and :497 (read)

Streaming body reads hardcode ImpitError::NetworkError, discarding the real cause:

let content = response.bytes().await.map(|b| b.to_vec()).map_err(|_| {
    ImpitPyError(impit::errors::ImpitError::NetworkError)
})?;

By contrast, the non-streaming from_async path correctly classifies the error via ImpitError::from(e, None).

Impact: a read-timeout mid-body, a truncated/UnexpectedEof response, or a gzip decode failure all surface to Python as a generic NetworkError with the original cause lost. Callers catching ReadTimeout / RemoteProtocolError / DecodingError will never match.

Suggested direction: use the same ImpitError::from(e, ...) classifier here as the non-streaming path.