Missing error handling on json.Unmarshal inside MySQL Row Iteration

Description

In the MySQL Datastore implementation, rowDataConverter.Data() blindly ignores the error returned by json.Unmarshal(jsonRaw, &obj).

If the datastore retrieves a malformed JSON slice or truncated bytes from MySQL, json.Unmarshal fails silently. Consequently, the function returns an empty or partially populated map. This leads to silent logic errors and "ghost bugs" down the pipeline instead of throwing an explicit error.

Code Context

https://github.com/pipe-cd/pipecd/blob/master/pkg/datastore/mysql/iterator.go#L86

Proposed Fix

Update the compiling interface (dataConverter) to return (map[string]interface{}, error) and bubble the failure up to the Iterator.Next() trace. I am raising a PR for this shortly.