Test fixture servers read accepted sockets as non-blocking on macOS #184

Problem

Several in-process test servers bind a listener, call set_nonblocking(true) on it, and read from each accepted stream with a read timeout. On Linux an accepted socket does not inherit O_NONBLOCK; on macOS and the BSDs it does. On macOS, read() returns WouldBlock immediately, the fixture parses an empty request, and it answers 404 {"message":"not found"}.

The tests are green in hosted CI (Linux) but fail deterministically for anyone running cargo test on a Mac. On lab2 (macOS arm64, Rust 1.86.0) on main bee4465:

tests/fixtures/replay-regressions/storage-paused-cold-completion.json: rust-storage-paused-cold-completion worker replay failed: http 404 Not Found: {"message":"not found"}

Under load more cases fail the same way. The same pattern explains the "flaky" lib tests seen locally on #182 and #183 (worker_session heartbeat lease, long-poll capacity backpressure, runtime-upload discovery, query worker, cooperative shutdown, the cold-worker corpus cases).

Evidence

Adding let _ = stream.set_nonblocking(false); right after accept() in tests/replay_regression_corpus.rs takes that suite from 27/28 to 28/28 locally.

Files that set a non-blocking listener: tests/replay_regression_corpus.rs, tests/sticky_server.rs, tests/worker_registration.rs, src/lib.rs (test module) and src/tests/runtime_uploads.rs.

Done when

Every accepted test stream is explicitly set to blocking before it is read, and the full cargo test --all-targets --all-features passes on macOS arm64 as well as in hosted CI. Test-only change; no product behavior changes.