## Summary
- stop reserving a localhost port in the websocket tests before spawning
the server
- let the app-server bind `127.0.0.1:0` itself and read back the actual
bound websocket address from stderr
- update the websocket test helpers and callers to use the discovered
address
## Why this fixes the flake
The previous harness reserved a port in the test process, dropped it,
and then asked the server process to bind that same address. On busy
runners there is a race between releasing the reservation and the child
process rebinding it, which can produce sporadic startup failures.
Binding to port `0` inside the server removes that race entirely, and
waiting for the server to report the real bound address makes the tests
connect only after the listener is actually ready.
