fix: use io.ReadFull instead of stream.Read

[gazzua]

What type of PR is this?

Bug fix

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What does this PR do? Why is it needed?

This PR fixes an issue in readContextFromStream where stream.Read could result in a partial read of the expected forkDigestLength bytes. When reading from a network stream, stream.Read does not guarantee returning all requested bytes in one call, leading to potential truncation or corruption of the context data (e.g., encountering unexpected 0x00 bytes seemed to prematurely "split" the data).

By replacing stream.Read with io.ReadFull, we ensure that:

• The full forkDigestLength bytes are read correctly.
• If the stream does not provide enough data, the function returns an error rather than silently returning incomplete data.

This change prevents any unexpected partial reads and ensures the fork digest is always read in full.

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Which issues(s) does this PR fix?

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Other notes for review

• The change is minimal but crucial for ensuring data integrity from the stream.

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Acknowledgements

• I have read CONTRIBUTING.md.
• I have included a uniquely named changelog fragment file.
• I have added a description to this PR with sufficient context for reviewers to understand this PR.

[prestonvanloon]

LGTM - This is a nice convenience wrapper too. Thanks!

Should we do this in all places of stream.Read?

    beacon-chain/sync/error.go|35 col 12| _, err := stream.Read(b)                                                                                                                                                 
    beacon-chain/sync/error.go|82 col 12| _, err := stream.Read(b)                                                                                                                                                 
    beacon-chain/sync/error.go|130 col 13| _, _err := stream.Read([]byte{0})                                                                                                                                       
    beacon-chain/sync/context_test.go|57 col 13| n, err := stream.Read(buf)                                                                                                                                        
    beacon-chain/sync/rpc_goodbye.go|120 col 13| _, _err := stream.Read([]byte{0})                                                                                                                                 
    beacon-chain/sync/rpc_beacon_blocks_by_range_test.go|152 col 14| _, err := stream.Read(b)                                                                                                                      
    beacon-chain/sync/rpc_beacon_blocks_by_range_test.go|271 col 14| _, err := stream.Read(b)

[gazzua]

LGTM - This is a nice convenience wrapper too. Thanks!

Should we do this in all places of stream.Read?

    beacon-chain/sync/error.go|35 col 12| _, err := stream.Read(b)                                                                                                                                                 
    beacon-chain/sync/error.go|82 col 12| _, err := stream.Read(b)                                                                                                                                                 
    beacon-chain/sync/error.go|130 col 13| _, _err := stream.Read([]byte{0})                                                                                                                                       
    beacon-chain/sync/context_test.go|57 col 13| n, err := stream.Read(buf)                                                                                                                                        
    beacon-chain/sync/rpc_goodbye.go|120 col 13| _, _err := stream.Read([]byte{0})                                                                                                                                 
    beacon-chain/sync/rpc_beacon_blocks_by_range_test.go|152 col 14| _, err := stream.Read(b)                                                                                                                      
    beacon-chain/sync/rpc_beacon_blocks_by_range_test.go|271 col 14| _, err := stream.Read(b)

@prestonvanloon
No, we don’t need to apply this change everywhere. The other cases only read 1 byte, so the current implementation is sufficient.