fix(conformance): runner_unreachable as a verdict is not a liveness failure

[jleni]

Fixes a regression I introduced in #392, found by the conformance leg on main
— run 36233876045.

What broke

#392 waited out any reply whose reason was runner_unreachable. Two
scenarios assert exactly that reason as the correct outcome:

first["state"] == "Unknown" && first["reason"] == "runner_unreachable"

• crash_before_spawn_is_unknown_and_never_retried
• crash_after_spawn_before_ack_is_unknown_and_runs_once

So the helper waited 60s for a verdict that was never going to change, then
failed with my own message:

[management] the runner never answered again after the injected crash cleared:
{"reason":"runner_unreachable","state":"Unknown","startedAt":…,"finishedAt":…}

Both passed before #392. I applied the helper to all four crash scenarios
because they shared a shape; only one of them shared the problem.

The two answers are distinguishable

	shape	meaning
verdict	state, startedAt, finishedAt	the execution is Unknown because the runner was unreachable
liveness	{"error": …, "reason": …}, no state	asked before the runner finished coming back; nothing recorded yet

#373's original failure was the second:
{"error":"the sandbox runner did not answer","reason":"runner_unreachable"}.
The ones above are the first. reason alone cannot tell them apart, and #392
keyed on reason alone.

Now keyed on the presence of state.

This is the class #399 documents

Merged the same day: the right construct — wait for the runner — pointing at
the wrong referent. reason looked like the thing that identified a liveness
failure and was not.

It is also the case for the rule in #399 about conformance: nothing in cargo test runs these scenarios, and a unit test over the helper would have used
whichever body shape I already believed in.

Verification

clippy --all-targets --all-features -D warnings clean, fmt clean. The
scenarios themselves only run on the conformance leg, so this needs
ci:conformance on the PR — which is the lever #399 says to pull and nobody
has.