feat(helm): reuse existing git sources in Kubernetes Helm-from-git install [BE-13046] (#2900)

Co-authored-by: Claude <noreply@anthropic.com>

api/http/handler/stacks/create_compose_stack_test.go

@@ -0,0 +1,31 @@
+package stacks
+
+import (
+	"testing"
+
+	portainer "github.com/portainer/portainer/api"
+	"github.com/stretchr/testify/assert"
+)
+
+func TestComposeGitPayload_ValidateWithSourceID_URLNotRequired(t *testing.T) {
+	t.Parallel()
+	payload := &composeStackFromGitRepositoryPayload{
+		Name:     "mystack",
+		SourceID: portainer.SourceID(1),
+		// RepositoryURL intentionally omitted
+	}
+
+	err := payload.Validate(nil)
+	assert.NoError(t, err)
+}
+
+func TestComposeGitPayload_ValidateWithoutSourceID_URLRequired(t *testing.T) {
+	t.Parallel()
+	payload := &composeStackFromGitRepositoryPayload{
+		Name: "mystack",
+		// SourceID and RepositoryURL both omitted
+	}
+
+	err := payload.Validate(nil)
+	assert.Error(t, err)
+}