server: skip LAN address tests when the host cannot reach it [no ci] (#32166)
The same-origin tests over the LAN address block forever on the Metal
runners: the request to the host's own LAN address never returns and
has no timeout, so the job runs until the 6h limit.
get_lan_ip() now probes /health on that address with a short timeout
and returns None when the host cannot reach it, which skips the test.
The test requests themselves use the default request timeout so a
hang fails the test instead of stalling the job.