[chore] Get only header of block

[RogerKSI]

Implementation details

• get only header of block instead of the whole block for block timestamp.

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[Copilot]

Pull request overview

This PR optimizes block data retrieval by fetching only block headers instead of full blocks when extracting block timestamps. This is a performance improvement that reduces network overhead and data transfer since only the header information is needed.

Changes:

• Replaced GetBlock method with GetHeaderBlock to retrieve only block headers
• Updated error messages and alert constants to reflect the new method name
• Updated mocks to match the new interface

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
relayer/chains/evm/provider.go	Updated to call GetHeaderBlock instead of GetBlock and access header.Time field directly
relayer/chains/evm/client.go	Renamed method from GetBlock to GetHeaderBlock, changed return type from *Block to *Header, and updated to use HeaderByNumber
relayer/alert/alert.go	Updated alert constant from GetBlockErrorMsg to GetHeaderBlockErrorMsg
internal/relayertest/mocks/chain_evm_client.go	Updated mock implementation to reflect the interface change from GetBlock to GetHeaderBlock

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