perf(import): do not check for existing grant ID (#8164)

# Which Problems Are Solved

Improve the performance of the `admin/v1/import` API endpoint.
Specifaclly the import of large amount of project grants.

# How the Problems Are Solved

`AddProjectGrantWithID` and `AddProjectGrantMember` methods of
`Commands` used to get the current state of the Writemodel to check if
the current GrantID or the combination of GrantID & UserID wasn't
already used. However, the Added events already have protection against
duplication by the `UniqueConstaint` methods.

The queries become very slow when there is a great amount of project
grants. Because all the events are pushed to the aggregate ID of the
project, we had to obtain all related project events, including events
of grantIDs we do not care about. This O(n) duration for bached import
jobs adding many organization granted to a single project.

This change removes the unnecesary state query to improve performance.

# Additional Changes

- Add integration tests for import

# Additional Context

- reported internally

internal/integration/assert.go

@@ -4,7 +4,10 @@ import (
 	"testing"
 	"time"
 
+	"github.com/pmezard/go-difflib/difflib"
 	"github.com/stretchr/testify/assert"
+	"google.golang.org/protobuf/encoding/protojson"
+	"google.golang.org/protobuf/proto"
 	"google.golang.org/protobuf/types/known/timestamppb"
 
 	object "github.com/zitadel/zitadel/pkg/grpc/object/v2beta"
@@ -71,3 +74,36 @@ func AssertListDetails[D ListDetailsMsg](t testing.TB, expected, actual D) {
 		assert.WithinRange(t, gotCD, wantCD.Add(-time.Minute), wantCD.Add(time.Minute))
 	}
 }
+
+// EqualProto is inspired by [assert.Equal], only that it tests equality of a proto message.
+// A message diff is printed on the error test log if the messages are not equal.
+//
+// As [assert.Equal] is based on reflection, comparing 2 proto messages sometimes fails,
+// due to their internal state.
+// Expected messages are usually with a vanilla state, eg only exported fields contain data.
+// Actual messages obtained from the gRPC client had unexported fields with data.
+// This makes them hard to compare.
+func EqualProto(t testing.TB, expected, actual proto.Message) bool {
+	t.Helper()
+	if proto.Equal(expected, actual) {
+		return true
+	}
+	t.Errorf("Proto messages not equal: %s", diffProto(expected, actual))
+	return false
+}
+
+func diffProto(expected, actual proto.Message) string {
+	diff, err := difflib.GetUnifiedDiffString(difflib.UnifiedDiff{
+		A:        difflib.SplitLines(protojson.Format(expected)),
+		B:        difflib.SplitLines(protojson.Format(actual)),
+		FromFile: "Expected",
+		FromDate: "",
+		ToFile:   "Actual",
+		ToDate:   "",
+		Context:  1,
+	})
+	if err != nil {
+		panic(err)
+	}
+	return "\n\nDiff:\n" + diff
+}