Returns the persisted ClaimOutput as a regenerated X12 837P file via
the new outbound serializer (Approach A — full rebuild from canonical
fields). 404 on missing claim, 422 if the stored raw_json cannot be
validated as a ClaimOutput. text/x12 content-type, attachment
disposition with the claim id as the filename.
3 tests:
- endpoint returns text/x12 attachment starting with ISA*
- 404 for missing claim id
- regenerated text round-trips back through parse()
Tyler