fix(bridge): support decoding final era1 file & fix broken merkle proofs for partial epochs

[njgheorghita]

What was wrong?

Era1 file #1896 is actually an "incomplete" era1 file, so our bridge was unable to deserialize it, as it expected all era1 files to be "complete".

We were also unable to generate valid merkle proofs for partial epochs.

How was it fixed?

Add support for deserializing era1 file 1896, but not other "incomplete" era1 files, since this is the only valid era1 file that we expect to be incomplete.

Tested locally against mainnet-01896-e6ebe562.era1 but the file is too large to include as a test asset.

Fixed broken merkle proof generation and added tests.

To-Do

• Clean up commit history and use conventional commits.

[KolbyML]

[morph-dev]

Few style related comments. Up to you if you want to apply them.

🚀

[morph-dev]

nit: Is it possible (and does it makes sense) to combine file.entries.len() == 21451 check with either hash, or index number or first/last block number of something like that?

Same in similar checks below.

[njgheorghita]

If I understand correctly, I'm not sure this is the place for that kind of verification.
The hash that this would be verified against does exists in the file path we provide, however there's no guarantee that this filepath is always the pointer provided for the file contents

• you can deserialize from a byte vector, so you wouldn't have a file path
• you can load an era1 file from a different filepath

So if there was to be any checking of the hash, it should be done by code/tooling a level above this module.
The same applies to index number & first/last block number. This code doesn't know what index/block number it's expected to find, so that validation should also occur a level above. I guess we could do some validation that the first_block_number = last_block_number - 8192, but I'm not sure if that's necessary? It's the kind of thing that would be covered if we're checking the root matches the expected hash a level above.

[morph-dev]

I think something like this would work:

let epoch_size = U256::from(epoch_acc.len());
let epoch_size = B256::from(epoch_size.to_le_bytes());
proof.push(epoch_size);

I would maybe even merge it in 2 lines, but that's less important.

[morph-dev]

I think you can extract header difficulty directly at this point:

let header_difficulty = B256::from(header_record.total_difficulty.to_le_bytes());

And you can also do it for header_record_hash:

let mut header_record_hash = B256::from(eth2_hashing::hash32_concat(
    header_record.block_hash.as_slice(),
    current_difficulty.as_slice(),
);

With above, you can even remove the loop:

let leaves = epoch_acc
    .iter()
    .map(|record| {
        B256::from(eth2_hashing::hash32_concat(
            record.block_hash.as_slice(),
            record.total_difficulty.as_le_slice(),
        ))
    })
    .collect::<Vec<_>>();

(or you can keep it as a loop, but much simpler)