Fix intermittent read hangs on reused HTTP/2 and pooled HTTP/1.1 connections

[benoitc]

HTTP/2 (primary)

A response that signals END_STREAM with a trailing HEADERS frame (trailers, or an empty trailing HEADERS, as proxies/ALBs emit for no-content-length streamed bodies) left the body reader parked forever. The h2 lib delivers it as a {trailers,...} event with no terminal DATA frame, and handle_h2_event ignored it, so the read blocked until the connection died (~30s) and returned nothing. h2-only because curl handles trailers fine.

This is not the issue-#544 socket active/passive path: h2 connections never traverse the pool is_ready/available socket path, they are reused via checkout_h2 and the h2 lib owns the socket.

Fix: route {trailers, StreamId, _} to the END_STREAM path so the read completes on fresh and reused connections.

Also adds a per-stream recv_timeout watchdog for sync h2 reads, which previously parked on an infinity gen_statem call with no timer, so any other lost-frame condition fails fast with {error, timeout} instead of hanging.

HTTP/1.1

The #544 idle close-detection sets pooled sockets to {active, once}, so data can land in the connection mailbox while idle. At checkout the old code discarded it with a best-effort receive ... after 0, which could strand bytes (passive recv blocks on an empty buffer) or corrupt the next read.

Fix: at checkout, drain the mailbox and peek the socket buffer, and refuse to reuse the connection on any pending data, close, or error. hackney does not pipeline, so idle data can never belong to the next response. Healthy idle connections still reuse normally, preserving keep-alive and the #544 stale-connection detection.

Tests

• hackney_http2_trailers_tests: deterministic trailered-response repro, a 40-request pooled-reuse stress alternating trailered and END_STREAM-on-DATA responses, and a never-ending response that must fail fast on recv_timeout.
• hackney_pool_tests: a connection fed unsolicited idle data is refused at checkout, while a clean idle connection stays reusable.

Full eunit suite (964 tests) and dialyzer pass.