fix(ids): unbound sockets are not listeners, and the test said otherwise

CI has been red on macOS since the IDS landed, through two commits, while I
reported "everything works" from a Windows run. Local green is not
verification and I wrote that down this morning.

The cause is one test of mine. Its doc comment says "not an assertion about
what it finds — that varies" and the next line asserts every listener has a
non-zero port. That is an assertion about what it finds. It is true on
Windows and false on macOS, where unbound sockets appear as `*.*` and parse
to port 0.

`current_listeners` now filters them, which is the right answer independently
of the test: nothing can connect to a socket with no port, and including them
would put entries in every baseline that differ between runs for reasons no
reader could act on.

The test now checks that invariant plus protocol non-emptiness, rather than a
habit of one platform.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>