Hide empty server slots (0.0.0.0) from list_servers output

Only show servers with actual IP addresses. Empty slots are
HAProxy pre-allocated placeholders that add noise to the output.

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

tests/unit/tools/test_servers.py

@@ -58,8 +58,8 @@ class TestHaproxyListServers:
 
             result = registered_tools["haproxy_list_servers"](domain="example.com")
 
+        assert "No active servers" in result
         assert "pool_1" in result
-        assert "disabled" in result
 
     def test_list_servers_with_active_servers(self, mock_socket_class, mock_select, patch_config_paths, response_builder):
         """List servers with active servers."""
@@ -90,7 +90,6 @@ class TestHaproxyListServers:
 
         assert "10.0.0.1" in result
         assert "10.0.0.2" in result
-        assert "active" in result
 
 
 class TestHaproxyAddServer: