fix(op-service/eth): Blob.UnmarshalText uses wrong hexutil label (Bytes32)

## Problem

`(*Blob).UnmarshalText` calls `hexutil.UnmarshalFixedText` with `"Bytes32"` even though `Blob` is a large fixed-size type (EIP-4844 blob), not 32 bytes. Error messages and validation can mislead callers.

## Expected

Use a type name consistent with `Blob`, e.g. `"Blob"`.

## PR

Tracked in the linked pull request.

Start in the op-service/eth package by locating (*Blob).UnmarshalText and inspect its call to hexutil.UnmarshalFixedText. Verify the resulting error and validation behavior, then confirm that the type label is consistent with Blob and run the relevant package tests.

Written by the indexing model from the issue text.