fix(op-service): CachingReceiptsProvider leaks fetching lock on FetchReceipts error

## Problem

`CachingReceiptsProvider.FetchReceipts` registers a per-block mutex in `fetching` and removes it after a successful fetch. If `inner.FetchReceipts` returns an error, the code returned without `deleteFetchingLock`, leaving the map entry (and mutex) behind indefinitely.

## Expected

Remove the fetching lock for the block hash on error, same as on success.

## PR

Tracked in the linked pull request.

Start at CachingReceiptsProvider.FetchReceipts and trace how the per-block fetching lock is registered and removed on the success path. Exercise the inner.FetchReceipts error path and verify that the lock for the block hash is also removed, with a regression test if the surrounding tests provide an appropriate location.

Written by the indexing model from the issue text.