Bug 25050: Adjust marc_records_to_documents to search entire mapping array

Bug 25050: Adjust marc_records_to_documents to search entire mapping array

There was an assumption in the ES code that match-heading mappings will appear in
a specified portion of the mappings array.

Certain mappings setups will not meet this assumption.
We need to move our searching up one level

The key seems to be having a mapping for a complete field, say 150, in both the
match-heading and another field as well as having mappings for ungrouped fields like
150a 150ab etc.

The unit test coverage should be sufficient for testing

Signed-off-by: Andrew Fuerste-Henry <andrew@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>