Changed to show filament price if no spool price set and added tests

tests_integration/tests/spool/test_add.py

@@ -185,3 +185,15 @@ def test_add_spool_both_used_and_remaining_weight(random_filament: dict[str, Any
         },
     )
     assert result.status_code == 400  # Cannot set both used and remaining weight
+
+def test_add_spool_price(random_filament: dict[str, Any]):
+    """Test adding a spool to the database."""
+    # Execute
+    result = httpx.post(
+        f"{URL}/api/v1/spool",
+        json={
+            "filament_id": random_filament["id"],
+            "price": 25,
+        },
+    )
+    assert result.status_code == 400  # Cannot set price