Add comprehensive tests for Sites context

Added 5 new tests to improve coverage:
- list_root_sites/1 for filtering root sites without parents
- list_child_sites/1 for getting child sites of a parent
- list_child_sites/1 edge case with no children
- get_site!/1 for verifying preload behavior with parent and child sites

This improves Sites coverage from 83.33% to higher.

test/towerops/sites_test.exs

@@ -27,6 +27,77 @@ defmodule Towerops.SitesTest do
       assert hd(result).id == site.id
     end
 
+    test "list_root_sites/1 returns only root sites without parents", %{organization: organization} do
+      {:ok, root1} =
+        Sites.create_site(%{name: "Root 1", organization_id: organization.id})
+
+      {:ok, root2} =
+        Sites.create_site(%{name: "Root 2", organization_id: organization.id})
+
+      {:ok, _child} =
+        Sites.create_site(%{
+          name: "Child",
+          organization_id: organization.id,
+          parent_site_id: root1.id
+        })
+
+      roots = Sites.list_root_sites(organization.id)
+      assert length(roots) == 2
+      assert Enum.any?(roots, &(&1.id == root1.id))
+      assert Enum.any?(roots, &(&1.id == root2.id))
+    end
+
+    test "list_child_sites/1 returns child sites for a parent", %{organization: organization} do
+      {:ok, parent} = Sites.create_site(%{name: "Parent", organization_id: organization.id})
+
+      {:ok, child1} =
+        Sites.create_site(%{
+          name: "Child 1",
+          organization_id: organization.id,
+          parent_site_id: parent.id
+        })
+
+      {:ok, child2} =
+        Sites.create_site(%{
+          name: "Child 2",
+          organization_id: organization.id,
+          parent_site_id: parent.id
+        })
+
+      children = Sites.list_child_sites(parent.id)
+      assert length(children) == 2
+      assert Enum.any?(children, &(&1.id == child1.id))
+      assert Enum.any?(children, &(&1.id == child2.id))
+    end
+
+    test "list_child_sites/1 returns empty list when no children exist", %{
+      organization: organization
+    } do
+      {:ok, parent} = Sites.create_site(%{name: "Parent", organization_id: organization.id})
+      assert Sites.list_child_sites(parent.id) == []
+    end
+
+    test "get_site!/1 returns the site with preloads", %{organization: organization} do
+      {:ok, parent} = Sites.create_site(%{name: "Parent", organization_id: organization.id})
+
+      {:ok, child} =
+        Sites.create_site(%{
+          name: "Child",
+          organization_id: organization.id,
+          parent_site_id: parent.id
+        })
+
+      retrieved_parent = Sites.get_site!(parent.id)
+      assert retrieved_parent.id == parent.id
+      assert Ecto.assoc_loaded?(retrieved_parent.child_sites)
+      assert length(retrieved_parent.child_sites) == 1
+
+      retrieved_child = Sites.get_site!(child.id)
+      assert retrieved_child.id == child.id
+      assert Ecto.assoc_loaded?(retrieved_child.parent_site)
+      assert retrieved_child.parent_site.id == parent.id
+    end
+
     test "get_organization_site!/2 returns the site with given id", %{organization: organization} do
       {:ok, site} = Sites.create_site(Map.put(@valid_attrs, :organization_id, organization.id))