op-e2e: Fix index in fakebeacon blob responses (#9408)

op-service: Fix error message when filtered number of sidecars doesn't match

op-e2e/e2eutils/fakebeacon/blobs.go

@@ -113,7 +113,7 @@ func (f *FakeBeacon) Start(addr string) error {
 				return
 			}
 			sidecars[i] = &eth.APIBlobSidecar{
-				Index:         eth.Uint64String(i),
+				Index:         eth.Uint64String(ix),
 				KZGCommitment: eth.Bytes48(bundle.Commitments[ix]),
 				KZGProof:      eth.Bytes48(bundle.Proofs[ix]),
 				SignedBlockHeader: eth.SignedBeaconBlockHeader{

op-service/sources/l1_beacon_client.go

@@ -140,7 +140,7 @@ func (cl *L1BeaconClient) GetBlobSidecars(ctx context.Context, ref eth.L1BlockRe
 	}
 
 	if len(hashes) != len(apiscs) {
-		return nil, fmt.Errorf("expected %v sidecars but got %v", len(hashes), len(resp.Data))
+		return nil, fmt.Errorf("expected %v sidecars but got %v", len(hashes), len(apiscs))
 	}
 
 	bscs := make([]*eth.BlobSidecar, 0, len(hashes))