The AI Gateway tests reserved a port with `testutil.RandomPort`, which
binds and closes `127.0.0.1:0`, then bound it later in `serve`.
If something took the port in between, `serve` failed at `net.Listen`
and the error went unread, leaving only a 10s `Eventually` timeout.
The gateway now exposes its bound address via `httpAddr` and a
`listenerReady` latch, so tests listen on port 0 and read the address
afterwards. `requireListening` reports `serve`'s error where an address
must still be fixed upfront.
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Investigated and authored with Coder Agents.
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Co-authored-by: Cian Johnston <cian@coder.com>
