Grab the port from bind_sockets in case its different

[choldgraf]

This asks a socket for the port that it has bound to, so that if the port changes (ie, when port=0 and the machine selects a new open port to return), it correctly makes it back into the build logs etc

Locally, I got this before the PR:

$ jupyter server --ServerApp.port=0 --no-browser
...
[I 2026-05-29 15:42:18.443 ServerApp] http://localhost:0/
...

And this after:

$ jupyter server --ServerApp.port=0 --no-browser
...
[I 2026-05-29 15:42:18.443 ServerApp] http://localhost:59976/
...

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• fixes Providing ServerApp.port=0 works but the server advertises port 0 instead of the bound port #1650

[krassowski]

LGTM!