fix(acp): include the relay response body in REST error messages

[Joxyko]

Summary

RestClient::request_with_retry builds its error from the status line alone and drops the response body:

return Err(RelayError::Http(format!(
    "{method} {} returned HTTP {}", path, resp.status()
)));

The relay explains which rule refused the request in that body — restricted: agent-turn-metric p tag must be the registered owner of this agent, restricted: not a relay member, and so on. The client discards it at the moment it arrives, so every caller upstack sees a bare 403 and has no way to tell an ownership problem from a membership problem from a rate-limit rule.

Concretely: a 403 on POST /events that blocked NIP-AM turn-metric publishing was unreadable from the client side for days. The reason was being sent correctly by the relay the whole time — the status code is the only thing that survived the client.

This appends the body to the message, bounded, on both the terminal arm and the retriable arm (last_err, which is what surfaces once retries are exhausted).

Behaviour when there is no body is unchanged — format_error_body returns an empty string, so the message is byte-for-byte what it was before. That is covered by a test, since it is the property most likely to be broken by a later edit.

Two things a reviewer will reasonably ask about, both handled:

• Unbounded bodies. This path is reached precisely when the peer is misbehaving, so the read itself is capped at 512 bytes via resp.chunk() rather than resp.text() — a failing request must not become a memory amplifier.
• Log injection. Control characters are collapsed to spaces, so a body cannot break one log line into several or forge a log entry. Truncation walks back to a UTF-8 char boundary.

Related issue

None found. Searched open PRs and issues touching crates/buzz-acp/src/relay.rs; the nearest neighbours are #5724 (honour the relay's retry hint on a rate-limited REST call) and #4912 (rate-limit overhaul … client retry fixes), which both change RestClient for other reasons — I checked their diffs and neither touches the Ok(resp) arms of request_with_retry, so this should not conflict. #4821 (recognize the membership gate's bodyless 404 in onboarding) is adjacent in spirit but is desktop-side and about a response that has no body at all.

Testing

• cargo test -p buzz-acp --lib → 797 passed, 0 failed (792 before, +5 new).
• cargo clippy -p buzz-acp --all-targets -- -D warnings → clean.
• cargo fmt -p buzz-acp -- --check → clean.
• Toolchain 1.95.0, the version pinned in rust-toolchain.toml.

New unit tests cover: empty body (message unchanged), the relay's real restricted: … reason coming through, newline flattening, the length bound, and a multi-byte character straddling the cap.