Getting this error on a non-PeerDAS network:
```
May 29 13:30:13.484 ERROR Error fetching or processing blobs from EL error: BlobProcessingError(AvailabilityCheck(Unexpected("empty blobs"))), block_root: 0x98aa3927056d453614fefbc79eb1f9865666d1f119d0e8aa9e6f4d02aa9395d9
```
It appears we're passing an empty `Vec` to DA checker, because all blobs were already seen on gossip and filtered out, this causes a `AvailabilityCheckError::Unexpected("empty blobs")`.
I've added equivalent unit tests for `getBlobsV1` to cover all the scenarios we test in `getBlobsV2`. This would have caught the bug if I had added it earlier. It also caught another bug which could trigger duplicate block import.
Thanks Santito for reporting this! 🙏
Addresses a regression recently introduced when we started gossip verifying data columns from EL blobs
```
failures:
network_beacon_processor::tests::accept_processed_gossip_data_columns_without_import
test result: FAILED. 0 passed; 1 failed; 0 ignored; 0 measured; 90 filtered out; finished in 16.60s
stderr ───
thread 'network_beacon_processor::tests::accept_processed_gossip_data_columns_without_import' panicked at beacon_node/network/src/network_beacon_processor/tests.rs:829:10:
should put data columns into availability cache: Unexpected("empty columns")
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
```
https://github.com/sigp/lighthouse/actions/runs/15309278812/job/43082341868?pr=7521
If an empty `Vec` is passed to the DA checker, it causes an unexpected error.
This PR addresses it by not passing an empty `Vec` for processing, and not spawning a task to publish.
#7461 and partly #6439.
Desired behaviour after receiving `engine_getBlobs` response:
1. Gossip verify the blobs and proofs, but don't mark them as observed yet. This is because not all blobs are published immediately (due to staggered publishing). If we mark them as observed and not publish them, we could end up blocking the gossip propagation.
2. Blobs are marked as observed _either_ when:
* They are received from gossip and forwarded to the network .
* They are published by the node.
Current behaviour:
- ❗ We only gossip verify `engine_getBlobsV1` responses, but not `engine_getBlobsV2` responses (PeerDAS).
- ❗ After importing EL blobs AND before they're published, if the same blobs arrive via gossip, they will get re-processed, which may result in a re-import.
1. Perform gossip verification on data columns computed from EL `getBlobsV2` response. We currently only do this for `getBlobsV1` to prevent importing blobs with invalid proofs into the `DataAvailabilityChecker`, this should be done on V2 responses too.
2. Add additional gossip verification to make sure we don't re-process a ~~blob~~ or data column that was imported via the EL `getBlobs` but not yet "seen" on the gossip network. If an "unobserved" gossip blob is found in the availability cache, then we know it has passed verification so we can immediately propagate the `ACCEPT` result and forward it to the network, but without re-processing it.
**UPDATE:** I've left blobs out for the second change mentioned above, as the likelihood and impact is very slow and we haven't seen it enough, but under PeerDAS this issue is a regular occurrence and we do see the same block getting imported many times.
Update `engine_getBlobsV2` response type to `Option<Vec<BlobsAndProofV2>>`. See recent spec change [here](https://github.com/ethereum/execution-apis/pull/630).
Added some tests to cover basic fetch blob scenarios.
