fix: don't emit the chunked terminator for zero-length body writes

[songhieu]

Fixes #935.

What

In ChunkedEncoding mode, BodyWriter encoded a zero-length application write as 0\r\n\r\n — which on the wire is the chunked terminator. finish() then wrote the terminator again, so the peer received the end-of-body sequence twice and parsed the second one as the start of a new message, desyncing the (keep-alive) connection.

This PR makes zero-length writes a no-op in chunked mode, in both write paths:

• do_write_chunked_body (async path): return Ok(Some(0)) without touching the stream.
• poll_write_chunked_body_task (cancel-safe task path): complete the task as Done(0) without writing.

finish() remains the only place that emits the terminator.

Why it matters in practice

proxy_h1.rs forwards the final downstream body chunk to the upstream even when it is empty (the mid-stream guard is deliberately !upstream_end_of_body && ...). An HTTP/2 downstream that ends the request body with an empty DATA frame carrying END_STREAM — Cloudflare's HTTP/2 to Origin does this for streamed POST bodies, as does curl -T - — therefore produces Body(Some(empty), end=true), and the H1 upstream receives:

Transfer-Encoding: chunked

11
{"hello":"world"}
0

0

Strict upstream parsers (e.g. uvicorn/h11) reject the duplicate terminator as a malformed pipelined request (400 Invalid HTTP request received.) and the poisoned pooled connection then serves that stale error to unrelated proxied requests. Full write-up with reproduction and captured wire bytes in #935.

Tests

• write_body_chunked_ignores_empty_chunk — async path: data chunk, empty write (asserts no wire bytes via the mock's exact-write expectations), single terminator on finish().
• write_body_task_chunked_ignores_empty_chunk — task path: same assertions through send_body_task / write_current_body_task.

cargo test -p pingora-core --lib protocols::http::v1::body passes.