fix(test): pin stdin so the net-scenario harness stops hanging

The stubbed ssh is `cat >/dev/null`, which drains stdin to EOF. Without an explicit stdin the stub inherits whatever the test runner had, so it returned instantly when stdin was redirected and blocked forever when it was a terminal or a live pipe. All five tests then hit their 20-second timeout.

That made the suite unreliable in the case that matters most, running it by hand. `make test-unit </dev/null` passed while the same target run from a terminal failed, which reads as a flaky test rather than an environment-dependent one.

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