fix: validate multicall calldata inputs

[silentgeckoaudit3801]

Closes #157.

Summary

• add isValidHexData for 0x-prefixed, byte-aligned raw calldata checks
• throw a typed ValidationError synchronously before aggregate3 payload construction when calldata is malformed
• cover missing prefix, odd-length calldata, invalid hex characters, and uppercase valid hex in Multicall tests

Validation

• vitest run tests/core/Multicall.test.ts (5 tests passed)
• git diff --check

[silentgeckoaudit3801]

Thanks for merging this. This and #161 cover the accepted #157 and #156 Whitechain SDK contributions.

Could you point me to the official GrantFox / Maybe Rewarded claim or payout step for these merged contributions? I?m not posting any private payment details publicly unless there is a documented process for that.