test(FN-4186): complete Step 2 — add chat prompt guidance regression tests

Fusion-Task-Id: FN-4186
Fusion-Task-Lineage: 30721f11-a830-4549-aa91-d8c3186c0dcf

packages/dashboard/src/__tests__/chat-system-prompt.test.ts

@@ -0,0 +1,24 @@
+import { describe, expect, it } from "vitest";
+import { CHAT_AGENT_MESSAGE_ROUTING_GUIDANCE, CHAT_SYSTEM_PROMPT } from "../chat.js";
+
+describe("chat system prompt guidance", () => {
+  it.each(["short", "crisp", "few sentences"])("includes brevity direction: %s", (token) => {
+    expect(CHAT_SYSTEM_PROMPT.toLowerCase()).toContain(token);
+  });
+
+  it("includes long-form follow-up path via fn_send_message", () => {
+    expect(CHAT_SYSTEM_PROMPT).toContain("fn_send_message");
+    expect(CHAT_SYSTEM_PROMPT).toContain('type: "agent-to-user"');
+    expect(CHAT_SYSTEM_PROMPT).toContain('to_id: "dashboard"');
+  });
+
+  it("combined guidance enforces additive mailbox follow-ups, not mirroring", () => {
+    const combined = `${CHAT_SYSTEM_PROMPT}\n\n${CHAT_AGENT_MESSAGE_ROUTING_GUIDANCE}`;
+
+    expect(combined).toContain("fn_send_message");
+    expect(combined).toContain('to_id: "dashboard"');
+    expect(combined.toLowerCase()).toContain("must not duplicate");
+    expect(combined.toLowerCase()).toContain("additive");
+    expect(combined.toLowerCase()).toContain("do not also call");
+  });
+});