fix: enhance hex to decimal conversion handling

[fukusuket]

What Changed

• Fixed array index access in hexadecimal conversion

Evidence

Integration-Test

• https://github.com/Yamato-Security/hayabusa/actions/runs/21484698296

I’d appreciate it if you could check it when you have time🙏

[Copilot]

Pull request overview

Improves HexToDecimal field conversion to be safer and more flexible when converting hex strings to decimal during field-data mapping.

Changes:

• Replaces fixed slicing ([2..]) with strip_prefix("0x"/"0X") to avoid panics on short strings.
• Adds explicit handling for empty hex payloads after the prefix (e.g., "0x"), falling back to the original string.

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[YamatoSecurity]

@fukusuket I checked that it works well. Thanks so much!!