test(headless): update v2 migration tests for fatal-by-default fallback policy

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/tests/headless-v2-migration.test.ts

@@ -66,6 +66,7 @@ class MockRpcClient {
 	sendUICalls: SendUICall[] = [];
 	initCalled = false;
 	initShouldFail = false;
+	stopCalled = false;
 
 	sendUIResponse(
 		id: string,
@@ -88,6 +89,26 @@ class MockRpcClient {
 		}
 		return { protocolVersion: 2 };
 	}
+
+	async stop(): Promise<void> {
+		this.stopCalled = true;
+	}
+}
+
+async function negotiateV2ForTest(
+	client: MockRpcClient,
+	allowLegacyFallback = false,
+): Promise<"v2" | "legacy" | "fatal"> {
+	try {
+		await client.init({ clientId: "sf-headless" });
+		return "v2";
+	} catch {
+		if (!allowLegacyFallback) {
+			await client.stop();
+			return "fatal";
+		}
+		return "legacy";
+	}
 }
 
 // ─── Extracted handleExtensionUIRequest (mirrors headless-ui.ts) ────────────
@@ -512,19 +533,24 @@ test("v2 init success sets v2Enabled", async () => {
 	assert.equal(v2Enabled, true);
 });
 
-test("v2 init failure falls back gracefully (v1 mode)", async () => {
+test("v2 init failure is fatal by default", async () => {
 	const client = new MockRpcClient();
 	client.initShouldFail = true;
-	let v2Enabled = false;
-	try {
-		await client.init({ clientId: "sf-headless" });
-		v2Enabled = true;
-	} catch {
-		// fall back to v1 — this is expected
-	}
+	const mode = await negotiateV2ForTest(client);
 
 	assert.equal(client.initCalled, true);
-	assert.equal(v2Enabled, false);
+	assert.equal(client.stopCalled, true);
+	assert.equal(mode, "fatal");
+});
+
+test("v2 init failure uses v1 only with explicit legacy fallback opt-in", async () => {
+	const client = new MockRpcClient();
+	client.initShouldFail = true;
+	const mode = await negotiateV2ForTest(client, true);
+
+	assert.equal(client.initCalled, true);
+	assert.equal(client.stopCalled, false);
+	assert.equal(mode, "legacy");
 });
 
 // ─── injector adapter ───────────────────────────────────────────────────────