hexToBytes silently converts invalid hex characters to zero bytes

`hexToBytes()` only checks for even length before parsing each byte with `parseInt(..., 16)`. Invalid hex pairs return `NaN`, and assigning `NaN` into a `Uint8Array` silently stores `0`.

File evidence:

- `src/utils.ts` strips an optional `0x` prefix and checks `hex.length % 2`.
- It then assigns `parseInt(`${hex[i]}${hex[i + 1]}`, 16)` into a `Uint8Array` without checking whether the result is `NaN`.
- The helper is used by `fromTurnkeySig()` for signature bytes and by `evmPkToCommitment()` for public key commitment derivation.

Examples like `0xZZZZ` or `0x12ZZ` therefore produce zero bytes instead of throwing. Invalid hex input should be rejected before it can affect signature or key material.