SprtDiscoveryService caught SocketException on the UDP 9100 bind and the
broadcast send and returned an empty list either way. That defeated the
CompositeDiscoveryService fix that rethrows only when every child discovery
service fails: on a real network fault the composite would see one child
throw (ENPC) and one child silently succeed empty (SPRT), conclude discovery
worked and found nothing, and let PrinterDiscoveryBackgroundTask expire and
deregister live Epson printers after the retention window - discarding their
queued jobs.
Move the policy decision to the composite, which already has it right: the
child now reports bind/broadcast failure honestly by rethrowing, and
CompositeDiscoveryService absorbs a single child's failure while still
rethrowing when every child fails. The per-datagram receive-loop catch is
unchanged - a single bad receive still shouldn't abort the pass.
Rewrote SprtDiscoveryServiceTests' bind-failure test to assert the throw, and
added an end-to-end CompositeDiscoveryServiceTests case with a real
SprtDiscoveryService failing to bind alongside a stub that succeeds, proving
the composite still degrades gracefully rather than losing the pass. Updated
the POS895 design doc to match the new contract.
Claude-Session: https://claude.ai/code/session_01E6N3P4nSZLtp4BC2LXVref
fd19cd9358