fix: display extra data as text instead of hex in test output

[fkondej]

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[claude]

Claude finished @fkondej's task in 53s —— View job

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Review of PR #387

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Summary: Clean, small fix that switches extra data display from hex to human-readable text and simplifies the comparison using bytes.Equal. One issue noted:

• Error message loses actual value — the error on mismatch now only shows the expected extra data, dropping the actual value. This makes debugging harder when the check fails. Left an inline suggestion to include both expected and actual values in the error message.