fix(test): classify() must survive a probe that answers with a status code

[codeslake]

The failure

CI red on Node 22, in refuses nothing when the proxy under it dies:

error: 'body.startsWith is not a function'
name: 'TypeError'

The cause

test/proxy-held-port.test.mjs has seven inline health probes. Six resolve `ERR:${e.code}` — a string carrying the prefix classify() tests for:

}).on("error", (e) => res(`ERR:${e.code}`));      // ×6

The seventh, at the forced-kill case, resolves a bare status code on success and a bare error code on failure:

r.resume(); r.on("end", () => res(r.statusCode));   // a Number
}).on("error", (e) => res(e.code));                 // a string, no prefix

Its caller filters out 200 and hands everything else straight to classify():

const cut = seen.filter((c) => c !== 200);
const refused = cut.filter((c) => classify(c) === OUTAGE.REFUSED);

So a 502 arrives as a Number and classify() dies on .startsWith, in a place where the correct answer is simply that was a reply, not an outage.

Why it took this long to surface

The path only opens when a non-200 is actually observed during that forced kill. Every local run and three CI matrices had none. That is also why it must be pinned by a unit case rather than left to the integration case that happens to reach it.

The change

if (typeof body !== "string") return null;

plus a unit case fixing the four answers:

input	classify
502	null — a status code is a reply, not an outage
200	null
"ERR:ECONNREFUSED"	OUTAGE.REFUSED
"ECONNRESET"	null — no ERR: prefix, not ours to classify

Measured: removing the type guard reds that unit case with the exact CI message, body.startsWith is not a function. Full file 35/35 with the guard.

Scope

Not caused by the branch that surfaced it — that branch touches neither this file nor classify(). Sent on its own so it can land independently.

Left alone deliberately: the seventh probe still resolves a different shape from the other six. Normalising it to `ERR:${e.code}` would change what the forced-kill case's seen.filter((c) => c !== 200) compares against, and that comparison is load-bearing for the assertion below it. The guard fixes the crash without moving that.

— Proxy Builder

🤖 Generated with Claude Code

[codeslake]

Folded into #345 as 392369b rather than landing separately. Same five-line guard and the same unit case; keeping it apart would have left #345's CI red on a crash it does not cause, needed a comment to explain that, and fixed the order the two had to merge in. Nothing here is abandoned — the fix is in #345.

— Proxy Builder

🤖 Generated with Claude Code