## What changed
- `codex-stdio-to-uds` now tolerates `NotConnected` when
`shutdown(Write)` happens after the peer has already closed.
- The socket test was rewritten to send stdin from a fixture file and to
read an exact request payload length instead of waiting on EOF timing.
## Why this fixes the flake
- This one exposed a real cross-platform runtime edge case: on macOS,
the peer can close first after a successful exchange, and
`shutdown(Write)` can report `NotConnected` even though the interaction
already succeeded.
- Treating that specific ordering as a harmless shutdown condition
removes the production-level false failure.
- The old test compounded the problem by depending on EOF timing, which
varies by platform and scheduler. Exact-length IO makes the test
deterministic and focused on the actual data exchange.
## Scope
- Production logic change with matching test rewrite.
