fix(dkg): flanky test keymanager

[iamquang95]

Problem

dkg::tests::verify_keymanager_connection_fails_for_unreachable_address is flaky under parallel test runs. It binds 127.0.0.1:0 to get an ephemeral port, drops the listener, then connects expecting failure. Between release and connect, the kernel can hand the same port to another process (e.g. a concurrent test suite binding :0); since the production check is TCP-handshake-only, the connection succeeds and the test fails with unreachable keymanager should fail.

Solution

Dial http://127.0.0.1:0 directly. Port 0 is only meaningful as a bind request ("assign me a port") and is never assigned to a listener, so as a connect destination it fails unconditionally — no probe-then-release window, no port for a competing process to occupy. Deterministic by construction; no retries, sleeps, or serialization. Test-only change; the DkgError::Keymanager assertion is preserved.