miner: fix block number extraction for BlockSigner tx

[gzliudan]

Proposed changes

Previously, the code used binary.BigEndian.Uint64(data[8:40]), which incorrectly read the index and only extracted the highest 8 bytes of the 32-byte left-padded block number, resulting in wrong values (often zero). Now, the code uses new(big.Int).SetBytes(data[4:36]).Uint64() to correctly extract the block number from the proper 32-byte field. This change fixes both the incorrect index and the parsing logic, ensuring accurate block number extraction and correct validation for special transactions.

eg:

when data is e341eaa4000000000000000000000000000000000000000000000000000000000000000f077c098bfe0729811df156252455c1debd1a8e4b7e781d8c7384ea32d15e5021

old(wrong):
data[8:40] is 0000000000000000000000000000000000000000000000000000000f077c098b
blkNumber is 0

new(right):
data[4:36] is 000000000000000000000000000000000000000000000000000000000000000f
blkNumber is 15

Types of changes

What types of changes does your code introduce to XDC network?
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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

Which part of the codebase this PR will touch base on,

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• Consensus
• Account
• Network
• Geth
• Smart Contract
• External components
• Not sure (Please specify below)

Checklist

Put an ✅ in the boxes once you have confirmed below actions (or provide reasons on not doing so) that

• 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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