Add the --disable-get-blobs-v2 flag and fixes #16171 (#16155)

**What type of PR is this?**
Feature + Bugfix

**What does this PR do? Why is it needed?**
Starting at Fusaka, the beacon node can pull blobs with the
`engine_getBlobsV2` API from the execution layer.
This reduces by a lot the burden on the beacon node. However, the beacon
node should be able to work 100% correctly without this execution layer
help.

This PR introduces the `--disable-get-blobs-v2` flag to simulate a 0%
success rate of this engine API.

This PR also fixes:
- https://github.com/OffchainLabs/prysm/issues/16171

Please read commit by commit with commit messages.

**How to test it:**
For the `--disable-get-blobs-v2` part:

Run the beacon node with the `--disable-get-blobs-v2` flag in DEBUG
mode.
For every block with commitments, the following log should be displayed:
```
[2025-12-19 15:36:25.49] DEBUG sync: No data column sidecars constructed from the execution client ...
```

And the following log should **never** be displayed:
```
[2026-01-05 10:19:00.55] DEBUG sync: Constructed data column sidecars from the execution client count=...
```

For the #16171 part:
- No ERROR log showed in the linked issue should never be displayed.

**Acknowledgements**
- [x] I have read
[CONTRIBUTING.md](https://github.com/prysmaticlabs/prysm/blob/develop/CONTRIBUTING.md).
- [x] I have included a uniquely named [changelog fragment
file](https://github.com/prysmaticlabs/prysm/blob/develop/CONTRIBUTING.md#maintaining-changelogmd).
- [x] I have added a description with sufficient context for reviewers
to understand this PR.
- [x] I have tested that my changes work as expected and I added a
testing plan to the PR description (if applicable).

beacon-chain/execution/BUILD.bazel

@@ -40,6 +40,7 @@ go_library(
         "//beacon-chain/state/state-native:go_default_library",
         "//beacon-chain/state/stategen:go_default_library",
         "//beacon-chain/verification:go_default_library",
+        "//cmd/beacon-chain/flags:go_default_library",
         "//config/fieldparams:go_default_library",
         "//config/params:go_default_library",
         "//consensus-types/blocks:go_default_library",

beacon-chain/execution/engine_client.go

@@ -11,6 +11,7 @@ import (
 	"github.com/OffchainLabs/prysm/v7/beacon-chain/core/peerdas"
 	"github.com/OffchainLabs/prysm/v7/beacon-chain/execution/types"
 	"github.com/OffchainLabs/prysm/v7/beacon-chain/verification"
+	"github.com/OffchainLabs/prysm/v7/cmd/beacon-chain/flags"
 	fieldparams "github.com/OffchainLabs/prysm/v7/config/fieldparams"
 	"github.com/OffchainLabs/prysm/v7/config/params"
 	"github.com/OffchainLabs/prysm/v7/consensus-types/blocks"
@@ -538,6 +539,10 @@ func (s *Service) GetBlobsV2(ctx context.Context, versionedHashes []common.Hash)
 		return nil, errors.New(fmt.Sprintf("%s is not supported", GetBlobsV2))
 	}
 
+	if flags.Get().DisableGetBlobsV2 {
+		return []*pb.BlobAndProofV2{}, nil
+	}
+
 	result := make([]*pb.BlobAndProofV2, len(versionedHashes))
 	err := s.rpcClient.CallContext(ctx, &result, GetBlobsV2, versionedHashes)