Harden sidecar startup failure handling

.core/TODO.md

@@ -1 +0,0 @@
-- @hardening pkg/display/sidecar.go:15 — `registerSidecarActions` ignores the error from `Start`, so a failed sidecar launch can still be treated as ready.

pkg/display/sidecar.go

@@ -12,7 +12,12 @@ import (
 func (s *Service) registerSidecarActions() {
 	if strings.TrimSpace(core.Env("CORE_DENO_ENABLE")) != "" && s.sidecar == nil {
 		s.sidecar = s.ensureSidecar()
-		_, _ = s.sidecar.Start(context.Background())
+		if _, err := s.sidecar.Start(context.Background()); err != nil {
+			if s != nil && s.ServiceRuntime != nil && s.Core() != nil {
+				s.Core().LogError(err, "display.registerSidecarActions", "failed to start enabled sidecar")
+			}
+			s.sidecar = nil
+		}
 	}
 
 	s.Core().Action("display.sidecar.start", func(ctx context.Context, _ core.Options) core.Result {

pkg/display/sidecar_test.go

@@ -58,6 +58,18 @@ func TestSidecar_EnsureSidecar_Ugly(t *testing.T) {
 	assert.Equal(t, "deno", manager.Status().Binary)
 }
 
+func TestSidecar_RegisterActions_StartFailureClearsSidecar(t *testing.T) {
+	t.Setenv("CORE_DENO_ENABLE", "1")
+	t.Setenv("CORE_DENO_BINARY", "/definitely/not/a/real/deno")
+
+	c := core.New(core.WithServiceLock())
+	svc := &Service{ServiceRuntime: core.NewServiceRuntime(c, Options{})}
+
+	svc.registerSidecarActions()
+
+	assert.Nil(t, svc.sidecar)
+}
+
 func TestSidecar_StatusAction_Good(t *testing.T) {
 	t.Setenv("CORE_DENO_BINARY", "/opt/core/deno")