fix(05-01): update health check API test for new JSON response

The enhanced health check now returns JSON with status/checks instead
of plain text. Update assertion to match.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: paultranvan

tests/api_tests/test_health.py

@@ -5,7 +5,8 @@ def test_health_check(api_client):
     """Test health check endpoint returns OK."""
     response = api_client.get("/health_check")
     assert response.status_code == 200
-    assert "RAG API is up" in response.text
+    data = response.json()
+    assert data["status"] in ("healthy", "degraded")
 
 
 def test_openapi_docs_accessible(api_client):