test(project): Cover build server recovery after non-abort errors

Pin the deadlock fix in BuildServer.#processBuildRequests with a
regression test: serve application.a in cache=Force mode against an
empty cache, fire two requests, and assert both reject with the
underlying "no cache found" error and that exactly two "error" events
are emitted. Before the fix, the second request hung until AVA's
timeout because #activeBuild was never cleared.

Author: RandomByte

packages/project/test/lib/build/BuildServer.integration.js

@@ -630,6 +630,40 @@ test.serial("Serve application.a with --cache=Force (2)", async (t) => {
 	await setTimeout(50);
 });
 
+// Regression: a non-abort build error used to leave #activeBuild set, deadlocking the BuildServer
+// so subsequent resource requests would hang forever. The fix in #processBuildRequests clears
+// #activeBuild in a finally block and emits "error" instead of throwing — verify a second request
+// still rejects (with the same root cause) instead of hanging.
+test.serial("Build server recovers from non-abort build error (no deadlock)", async (t) => {
+	const fixtureTester = t.context.fixtureTester = await FixtureTester.create(t, "application.a");
+
+	const errorEvents = [];
+	await fixtureTester.serveProject({config: {cache: Cache.Force}, expectBuildErrors: true});
+	fixtureTester.buildServer.on("error", (err) => errorEvents.push(err));
+
+	// First request triggers a build that fails because cache=Force has no cache
+	const firstError = await t.throwsAsync(async () => {
+		await fixtureTester.requestResource({resource: "/test.js"});
+	});
+	t.true(
+		firstError.message.includes(`Cache is in "Force" mode but no cache found for project application.a`),
+		"First request rejects with the Force-mode cache miss"
+	);
+
+	// Second request must reject again (not hang) — proves #activeBuild was cleared after the failure
+	const secondError = await t.throwsAsync(async () => {
+		await fixtureTester.requestResource({resource: "/test.js"});
+	});
+	t.true(
+		secondError.message.includes(`Cache is in "Force" mode but no cache found for project application.a`),
+		`Second request rejects with the same error instead of deadlocking. Got: ${secondError && secondError.message}`
+	);
+
+	// Each failed build emits exactly one "error" event
+	await setTimeout(50);
+	t.is(errorEvents.length, 2, "Two build errors were emitted, one per failed build attempt");
+});
+
 // ProjectBuildCache's StageCache must be cleared correctly when a build is aborted.
 // A task that completed during an aborted attempt has already called recordTaskResult,
 // which adds its stage to the in-memory StageCache. On retry,