Add Handler Validation Before Data Migration

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### Preconditions

1. Use of a customised map.xml.

### Steps to reproduce

1. Modify map.xml to reference a handler class that doesn't exist (e.g. ``).
2. Run the data migration `bin/magento migrate:data -r config.xml`

### Expected result

1. Error before any data is copied, so you don't end up in a broken state.

### Actual result

1. Error at the point it attempts to invoke that handler, this can be after a long period of time, leaving the DB in a half migrated state. Currently this leads to a very slow feedback loop to realise you've made a typo in a class name.

### Additional notes

Start with the data migration entry point `bin/magento migrate:data -r config.xml` and trace how the customised `map.xml` handler class is resolved. Verify where the handler is first invoked, then ensure an invalid class is reported before any data is copied; confirm the migration no longer reaches a partial state.

Written by the indexing model from the issue text.