HTTP response collection failures currently collapse the upstream body to empty

[GuthL]

Summary

When KeyClaw intercepts a non-SSE HTTP response and body.collect().await fails, the handler currently returns the upstream response headers with Body::empty().

Evidence

• src/proxy/http.rs:252-255

let (mut parts, body) = res.into_parts();
let collected = match body.collect().await {
    Ok(collected) => collected,
    Err(_) => return Response::from_parts(parts, Body::empty()),
};

Why this matters

A transient upstream read error or truncated body should not silently turn into a successful-looking empty response. That corrupts the client-visible result and makes debugging much harder because the original failure mode is discarded.

Suggested scope

• Preserve or surface the upstream read failure instead of swapping in an empty body.
• Add a regression test for the error path so intercepted responses do not degrade into silent empties.
• Keep placeholder-resolution behavior unchanged on the successful path.