Add tests for release channel resolution and caching

Cover fetchLatestReleases and resolveReleaseChannel, the controller's
latest-releases cache with its on-disk offline fallback, the
LatestReleasesRequest round-trip, and FactorioServer.setTargetVersion.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

test/lib/external/LatestReleases.test.js

@@ -0,0 +1,88 @@
+const assert = require("assert").strict;
+
+const { fetchLatestReleases, resolveReleaseChannel } = require("@clusterio/lib");
+const { slowTest } = require("../../integration");
+
+describe("LatestReleases", function() {
+	describe("fetchLatestReleases", function() {
+		const originalFetch = global.fetch;
+
+		beforeEach(() => {
+			global.fetch = undefined;
+		});
+
+		afterEach(() => {
+			global.fetch = originalFetch;
+		});
+
+		const sample = {
+			experimental: { alpha: "2.1.8", demo: "2.0.77", expansion: "2.1.8", headless: "2.1.8" },
+			stable: { alpha: "2.0.77", demo: "2.0.77", expansion: "2.0.77", headless: "2.0.77" },
+		};
+
+		function mockFetch(json) {
+			global.fetch = async () => ({
+				ok: true,
+				status: 200,
+				statusText: "OK",
+				json: async () => json,
+			});
+		}
+
+		it("fetches and returns the releases json", async function() {
+			mockFetch(sample);
+
+			const releases = await fetchLatestReleases();
+			assert.deepEqual(releases, sample);
+		});
+		it("throws on HTTP error status", async function() {
+			global.fetch = async () => ({
+				ok: false,
+				status: 500,
+				statusText: "Server Error",
+				json: async () => ({}),
+			});
+
+			await assert.rejects(fetchLatestReleases(), /HTTP 500 Server Error/);
+		});
+		it("throws on network error", async function() {
+			global.fetch = async () => {
+				throw new Error("Network Error");
+			};
+
+			await assert.rejects(fetchLatestReleases(), /Network Error/);
+		});
+		it("fetches from the live api", async function() {
+			slowTest(this);
+			global.fetch = originalFetch;
+
+			const releases = await fetchLatestReleases();
+			assert.ok(releases.stable, "expected a stable channel");
+			assert.ok(releases.experimental, "expected an experimental channel");
+			assert.equal(typeof releases.stable.headless, "string");
+		});
+	});
+
+	describe("resolveReleaseChannel", function() {
+		const releases = {
+			experimental: { alpha: "2.1.8", demo: "2.0.77", headless: "2.1.8" },
+			stable: { alpha: "2.0.77", headless: "2.0.77" },
+			// A channel without a headless build, to exercise the fallback.
+			oddball: { alpha: "1.2.3" },
+		};
+
+		it("returns the headless build version by default", function() {
+			assert.equal(resolveReleaseChannel(releases, "stable"), "2.0.77");
+			assert.equal(resolveReleaseChannel(releases, "experimental"), "2.1.8");
+		});
+		it("returns the requested build version", function() {
+			assert.equal(resolveReleaseChannel(releases, "experimental", "demo"), "2.0.77");
+		});
+		it("falls back to the first build when headless is missing", function() {
+			assert.equal(resolveReleaseChannel(releases, "oddball"), "1.2.3");
+		});
+		it("returns undefined for an unknown channel", function() {
+			assert.equal(resolveReleaseChannel(releases, "nightly"), undefined);
+		});
+	});
+});