refactor: implement core and server dir to split the library and endpoints for testing

tests/test_server_grade_1.py

@@ -0,0 +1,34 @@
+import unittest
+
+from fastapi.testclient import TestClient
+
+from math_problems_structure.server.app import app
+
+
+class Grade1ServerTest(unittest.TestCase):
+    def test_join_pictures_with_quantity_route_uses_core_generator(self) -> None:
+        client = TestClient(app)
+        response = client.post(
+            "/grade-1/join-pictures-with-quantity",
+            json={
+                "available_pictures": [
+                    {
+                        "id": f"picture-{index}",
+                        "name": f"Picture {index}",
+                        "image_path": f"/images/{index}.png",
+                    }
+                    for index in range(10)
+                ],
+                "seed": 1,
+            },
+        )
+
+        self.assertEqual(response.status_code, 200)
+        problem = response.json()
+        self.assertEqual(len(problem["left_containers"]), 5)
+        self.assertEqual(len(problem["number_cards"]), 5)
+        self.assertEqual(len(problem["right_containers"]), 5)
+
+
+if __name__ == "__main__":
+    unittest.main()