consensus: fix missing block header error in vote verification, fix #1850

[gzliudan]

Proposed changes

This PR eliminates the following errors which are produced by PR #1850:

ERROR[12-23|23:05:41.035] [VerifyVoteMessage] Fail to get epochInfo when verifying vote message! blockNum=76,854,127 blockHash=147610..416730 voteHash=4d0a5c..9e7fcf voteGapNumber=76,852,350 err="[getEpochSwitchInfo] can not find header from db hash 0x147610f021c9b4b5fda55239a807181c52c864a3b25d02ba0b9785105a416730"
ERROR[12-23|23:05:41.035] [Vote] Verify BFT Vote                   error="[getEpochSwitchInfo] can not find header from db hash 0x147610f021c9b4b5fda55239a807181c52c864a3b25d02ba0b9785105a416730"

ERROR[12-23|23:05:43.380] [VerifyVoteMessage] Fail to get epochInfo when verifying vote message! blockNum=76,854,128 blockHash=6eaf17..9feb10 voteHash=54c559..dafbde voteGapNumber=76,852,350 err="[getEpochSwitchInfo] can not find header from db hash 0x6eaf1777cab5f487090909afd5eabf15dd103413a833a305b1b8970d349feb10"
ERROR[12-23|23:05:43.380] [Vote] Verify BFT Vote                   error="[getEpochSwitchInfo] can not find header from db hash 0x6eaf1777cab5f487090909afd5eabf15dd103413a833a305b1b8970d349feb10"

Types of changes

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• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation Update (if none of the other choices apply)
• Regular KTLO or any of the maintaince work. e.g code style
• CICD Improvement

Impacted Components

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• Consensus
• Account
• Network
• Geth
• Smart Contract
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Checklist

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• This PR has sufficient test coverage (unit/integration test) OR I have provided reason in the PR description for not having test coverage
• Provide an end-to-end test plan in the PR description on how to manually test it on the devnet/testnet.
• Tested the backwards compatibility.
• Tested with XDC nodes running this version co-exist with those running the previous version.
• Relevant documentation has been updated as part of this PR
• N/A

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[Copilot]

Pull request overview

This PR addresses spurious error logs that occur when vote messages arrive before their corresponding block headers during normal network operations. The fix adds an early check to defer vote verification when the referenced header is not yet available locally, preventing misleading error messages and allowing the vote to be retried once the header arrives.

Key Changes:

• Added header presence check before epoch info lookup in vote verification
• Changed logging from ERROR to DEBUG level for the normal timing scenario where votes arrive before headers

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[Copilot]

The new header check successfully prevents spurious error logs, but this code path lacks test coverage. Consider adding a test case in vote_test.go that verifies the behavior when a vote arrives before its corresponding block header is available. The test should confirm that the function returns (false, nil) and logs at Debug level rather than Error level when the header is not found.