Fix 404 handling when deleting a manifest (#850)

Upstream was previously using an internal notFound error, and we were
relying on fragile string matching to detect it. This broke when we
upgraded to Docker v28.

d25e260d2e (diff-a5e122cd2318f2dc156f373804a59d30355b0c308b9e64f48e0713344fcdba33L164)

Nowadays containerd exposes a public ErrNotFound which we can use
instead.

Fixes #849.

provider/internal/index_test.go

@@ -24,6 +24,7 @@ import (
 	"go.uber.org/mock/gomock"
 
 	provider "github.com/pulumi/pulumi-go-provider"
+	"github.com/pulumi/pulumi-go-provider/infer"
 	"github.com/pulumi/pulumi-go-provider/integration"
 	"github.com/pulumi/pulumi/sdk/v3/go/common/resource"
 	"github.com/pulumi/pulumi/sdk/v3/go/common/util/mapper"
@@ -235,3 +236,26 @@ func TestIndexDiff(t *testing.T) {
 		})
 	}
 }
+
+func TestIndexDelete(t *testing.T) {
+	t.Parallel()
+	t.Run("manifest already deleted (404)", func(t *testing.T) {
+		t.Parallel()
+		ctrl := gomock.NewController(t)
+		client := NewMockClient(ctrl)
+		client.EXPECT().
+			ManifestDelete(gomock.Any(), "docker.io/pulumi/test:manifest").
+			Return(errNotFound{})
+
+		i := &Index{clientF: mockClientF(client)}
+
+		_, err := i.Delete(t.Context(), infer.DeleteRequest[IndexState]{
+			ID: "foo",
+			State: IndexState{
+				IndexArgs: IndexArgs{Tag: "docker.io/pulumi/test:manifest"},
+				Ref:       "docker.io/pulumi/test:manifest",
+			},
+		})
+		assert.NoError(t, err)
+	})
+}