feat(FN-4400): complete Step 7 — add prompt-size api and dashboard sparkline

Fusion-Task-Id: FN-4400
Fusion-Task-Lineage: 0d4f9c48-5b8b-49eb-9cf3-d1d585ffe7fc

packages/dashboard/src/__tests__/routes-agent-prompt-sizes.test.ts

@@ -0,0 +1,98 @@
+import { describe, it, expect, vi, beforeEach } from "vitest";
+import { EventEmitter } from "node:events";
+import { get } from "../test-request.js";
+import { createServer } from "../server.js";
+
+const {
+  mockInit,
+  mockGetAgent,
+  mockIsEphemeralAgent,
+  mockChatStoreInit,
+  mockAll,
+} = vi.hoisted(() => ({
+  mockInit: vi.fn().mockResolvedValue(undefined),
+  mockGetAgent: vi.fn(),
+  mockIsEphemeralAgent: vi.fn(),
+  mockChatStoreInit: vi.fn().mockResolvedValue(undefined),
+  mockAll: vi.fn(),
+}));
+
+vi.mock("@fusion/core", () => ({
+  AgentStore: class MockAgentStore {
+    init = mockInit;
+    getAgent = mockGetAgent;
+  },
+  isEphemeralAgent: mockIsEphemeralAgent,
+  ChatStore: class MockChatStore {
+    init = mockChatStoreInit;
+  },
+}));
+
+class MockStore extends EventEmitter {
+  getRootDir(): string {
+    return "/tmp/fn-4400-test";
+  }
+
+  getFusionDir(): string {
+    return "/tmp/fn-4400-test/.fusion";
+  }
+
+  getDatabase() {
+    return {
+      exec: vi.fn(),
+      prepare: vi.fn().mockReturnValue({
+        run: vi.fn().mockReturnValue({ changes: 0 }),
+        get: vi.fn(),
+        all: mockAll,
+      }),
+    };
+  }
+}
+
+describe("GET /api/agents/:id/prompt-sizes", () => {
+  beforeEach(() => {
+    vi.clearAllMocks();
+    mockGetAgent.mockResolvedValue({ id: "agent-001", role: "executor", metadata: {} });
+    mockIsEphemeralAgent.mockReturnValue(false);
+    mockAll.mockReturnValue([
+      {
+        runId: "run-1",
+        createdAt: "2026-05-14T00:00:00.000Z",
+        systemChars: 120,
+        execChars: 880,
+        totalChars: 1000,
+      },
+      {
+        runId: "run-2",
+        createdAt: "2026-05-13T23:00:00.000Z",
+        systemChars: 0,
+        execChars: 0,
+        totalChars: 0,
+      },
+    ]);
+  });
+
+  it("returns recent prompt size rows", async () => {
+    const app = createServer(new MockStore() as any);
+    const res = await get(app, "/api/agents/agent-001/prompt-sizes");
+    expect(res.status).toBe(200);
+    expect(res.body).toEqual([
+      expect.objectContaining({ runId: "run-1", totalChars: 1000 }),
+      expect.objectContaining({ runId: "run-2", systemChars: 0, execChars: 0, totalChars: 0 }),
+    ]);
+  });
+
+  it("returns 404 when agent is missing", async () => {
+    mockGetAgent.mockResolvedValueOnce(null);
+    const app = createServer(new MockStore() as any);
+    const res = await get(app, "/api/agents/missing/prompt-sizes");
+    expect(res.status).toBe(404);
+  });
+
+  it("returns 400 for ephemeral agents", async () => {
+    mockIsEphemeralAgent.mockReturnValueOnce(true);
+    const app = createServer(new MockStore() as any);
+    const res = await get(app, "/api/agents/agent-001/prompt-sizes");
+    expect(res.status).toBe(400);
+  });
+});