code: def test_router_a_a():
      with client:
          response = client.get("/a/a")
      assert response.json() == {"msg": "Hello A A"}
      assert response.headers["content-type"] == json_type
  
imports: from fastapi import APIRouter, FastAPI
  from fastapi.responses import HTMLResponse, JSONResponse, PlainTextResponse
  from fastapi.testclient import TestClient
label_desc: Use image node_id 19 for calling a global function with example usage: test_router_a_a() without return types