Bug 14697: (follow-up) Rely on the UNIQUE constraint and return 409 for issue_id

Bug 14697: (follow-up) Rely on the UNIQUE constraint and return 409 for issue_id

This patch avoids querying the DB for an already existing
Koha::Checkouts::ReturnClaim with the same issue_id, now that there's a
UNIQUE constraint on it.

Also, 409 should be returned instead. Tests added for this changes.

To test:
- Apply this patch
- Run:
  $ kshell
 k$ prove t/db_dependent/api/v1/return_claims.t
=> SUCCESS: tests pass!

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Andrew Fuerste-Henry <andrew@bywatersolutions.com>
Signed-off-by: Lisette Scheer <lisetteslatah@gmail.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>