Fix PinghubWebSocketTests port collisions

[jkmassel]

Fixes PinghubWebSocketTests.testReceiveManyMessage (62% reliable over the last 7 days in Test Engine, labeled flaky). The same bug affects every test in the class; testReceiveManyMessage is just the one that loses most often on CI.

Root Cause

PinghubServer picked a random port in 9000...9999 and retried up to 5 times if WebSocketServer.start returned an error. But Starscream's start creates an NWListener and returns nil right away — the bind happens asynchronously — so a taken port is never reported and the retry loop never retries:

nw_listener_socket_inbox_create_socket bind(83, ::.9201) ... failed [48: Address already in use]
nw_read_request_report [C147] Receive failed with error "Connection reset by peer"

The port is usually taken by a listener from an earlier test. PinghubServer never stops its listener (Starscream 4.0.8 has no API for it), so listeners pile up for the life of the test process. When a new server lands on one of their ports, the client connects to the stale listener, the connection gets reset, and the test waits out its 20s timeout.

Reproduced locally by running PinghubWebSocketTests 30× in one process: 11/150 executions failed, and 22 of the 150 servers hit Address already in use.

Separately, connect() only waited for the client to see the upgrade response. The server appends the connection to clients just after sending that response, on its own queue, so a broadcast could in principle race it (and clients was read and written from two threads).

Fix

• Ask the OS for a free port. unusedPort() binds a dual-stack socket to port 0, reads the assigned port with getsockname, and releases it for the server. Leaked listeners still hold their ports, so the OS won't hand those out.
• Wait for both ends in connect(). The server fulfills a "registered" expectation once it has the connection, so broadcast never runs against an empty clients list.
• Keep clients on the main thread. Server events hop to main before touching the list.

Test-only; PinghubClient is unchanged.

Test plan

• Local: PinghubWebSocketTests × 100 iterations in one process (Xcode 27.0, iOS 27.0 simulator) — 500/500 passed, 0 Address already in use, 2.8s total (vs 190s with the timeouts)
• CI: stress builds, each on its own ref at this commit; check Test Engine first-attempt executions

[dangermattic]

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