fix(deserialize): overflow validation for uint64 fields

[yevshev]

Summary

This PR adds uint64 overflow validation during deserialization to prevent silent truncation from malicious RLP payloads.

Changes

• Add uint64 overflow validation to gas, nonce, validBefore, and validAfter fields
• Extract repeated big.Int to uint64 conversion and validation into a shared bytesToUint64 helper
• Expand TestDeserialize_OversizedSignatures into TestDeserialize_OversizedFields to also cover all uint64 fields

Testing

• All Existing transaction package tests pass
• New oversized field test cases assert correct error messages for gas, nonce, validBefore, validAfter

Closes #17