fix(proxy): ignore empty body payloads in HTTP/1 upstream writes

[Curious-r]

Summary

Fix a spurious HTTP/1 warning when an empty final body task follows a fully written Content-Length body. The upstream sink now skips empty payloads in both Body and UpgradedBody, while finish_body() still handles final framing and Content-Length validation. Non-empty data after the body is finished continues to trigger the warning.

Added regression tests for Content-Length and chunked bodies to ensure empty final tasks don't affect wire framing or produce duplicate chunk terminators.

Validated with cargo test -p pingora-proxy --lib, cargo clippy -p pingora-proxy --lib --tests -- -D warnings, and cargo fmt --all -- --check.

Background:

When using a Pingora-based reverse proxy for my Matrix homeserver, I noticed that it produced a large number of warnings like:

WARN pingora_proxy::proxy_h1: Upstream body is already finished. Nothing to write

After enabling DEBUG logging, the warning could be reproduced on normal Matrix federation requests. The relevant sequence was:

DEBUG pingora_proxy::proxy_h1: Read 208 bytes body from downstream
DEBUG pingora_proxy::proxy_h1: Read 0 bytes body from downstream
DEBUG pingora_proxy::proxy_h1: Write 208 bytes body to upstream
WARN  pingora_proxy::proxy_h1: Upstream body is already finished. Nothing to write
DEBUG pingora_proxy::proxy_h1: finish sending body to upstream

This shows that the warning is triggered by an empty body event arriving after the complete Content-Length body has already been written. Since empty payloads do not carry any data and finish_body() already handles the end-of-body framing, the empty buffer should be ignored rather than passed to write_body().