add basic systemd service monitoring (#1153)

Co-authored-by: Shelby Tucker <shelby.tucker@gmail.com>

agent/systemd_nonlinux_test.go

@@ -0,0 +1,53 @@
+//go:build !linux && testing
+
+package agent
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestNewSystemdManager(t *testing.T) {
+	manager, err := newSystemdManager()
+	assert.NoError(t, err)
+	assert.NotNil(t, manager)
+}
+
+func TestSystemdManagerGetServiceStats(t *testing.T) {
+	manager, err := newSystemdManager()
+	assert.NoError(t, err)
+
+	// Test with refresh = true
+	result := manager.getServiceStats(true)
+	assert.Nil(t, result)
+
+	// Test with refresh = false
+	result = manager.getServiceStats(false)
+	assert.Nil(t, result)
+}
+
+func TestSystemdManagerGetServiceDetails(t *testing.T) {
+	manager, err := newSystemdManager()
+	assert.NoError(t, err)
+
+	result, err := manager.getServiceDetails("any-service")
+	assert.Error(t, err)
+	assert.Equal(t, "systemd manager unavailable", err.Error())
+	assert.Nil(t, result)
+
+	// Test with empty service name
+	result, err = manager.getServiceDetails("")
+	assert.Error(t, err)
+	assert.Equal(t, "systemd manager unavailable", err.Error())
+	assert.Nil(t, result)
+}
+
+func TestSystemdManagerFields(t *testing.T) {
+	manager, err := newSystemdManager()
+	assert.NoError(t, err)
+
+	// The non-linux manager should be a simple struct with no special fields
+	// We can't test private fields directly, but we can test the methods work
+	assert.NotNil(t, manager)
+}