Change u.trajectory.n_atoms to u.atoms.n_atoms as requested

Co-authored-by: ljwoods2 <145226270+ljwoods2@users.noreply.github.com>

Author: Copilot

imdclient/tests/test_imdclient.py

@@ -99,7 +99,7 @@ def server_client_incorrect_atoms(self, universe, imdsinfo, port):
         client = IMDClient(
             f"localhost",
             port,
-            universe.trajectory.n_atoms + 1,
+            universe.atoms.n_atoms + 1,
         )
         server.join_accept_thread()
         yield server, client
@@ -192,8 +192,8 @@ def test_incorrect_atom_count(self, server_client_incorrect_atoms, universe):
             client.get_imdframe()
 
         error_msg = str(exc_info.value)
-        assert f"Expected n_atoms value {universe.trajectory.n_atoms + 1}" in error_msg
-        assert f"got {universe.trajectory.n_atoms}" in error_msg
+        assert f"Expected n_atoms value {universe.atoms.n_atoms + 1}" in error_msg
+        assert f"got {universe.atoms.n_atoms}" in error_msg
         assert "Ensure you are using the correct topology file" in error_msg