Are there any assertions you want to make here about either the request or the response?

The response does not have any public fields. So, there is nothing to assert.
https://github.com/googleapis/go-genproto/blob/50ed04b929172f52f906d64b8689663a5afbc9e2/googleapis/datastore/v1/datastore.pb.go#L1374-L1378

Are there any assert statements that could be added to make sure the code did "prevent the supplied keys' IDs from being auto-allocated by Cloud Datastore"? Perhaps by making a read call against the keys and ensuring the result of the read is nil?

The result of read is nil with and without reserving IDs.

I see. Maybe I misunderstood the meaning of "prevent the supplied keys' IDs from being auto-allocated by Cloud Datastore". Is there any test with an assert statement that ensures calling reserveIds will "prevent the supplied keys' IDs from being auto-allocated by Cloud Datastore"? Is there anything we are doing to ensure that behaviour is captured?

I could add a test like this but 1000 operations does not exhaust the possibility that the id could be allocated later. So, not a definite test:

	reservedKeyIDs := []int64{1234567, 8900000, 7654897}
	keys := make([]*Key, len(reservedKeyIDs))
	for i := range reservedKeyIDs {
		keys[i] = IDKey("ReserveIDs", reservedKeyIDs[i], nil)
	}
	err := client.ReserveIDs(ctx, keys)
	if err != nil {
		t.Fatalf("ReserveIDs failed: %v", err)
	}

	type value struct{ N int }
	v := &value{N: 5}
	for i := 0; i < 1000; i++ {
		autoAllocatedKey, err := client.Put(ctx, IncompleteKey("ReserveIDs", nil), v)
		if err != nil {
			t.Fatal(err)
		}

		if slices.Contains(reservedKeyIDs, autoAllocatedKey.ID) {
			t.Fatalf("Reserved ID auto-allocated by Datastore: %v\n", autoAllocatedKey.ID)
		}
	}

Ideally, Datastore service should return an error if reservation failed. So, just checking for error should suffice, right?

Ok. I see your point and understand that this could be very difficult to test since the allocated key for client.Put is basically random. This LGTM.