Nice pictures from S France! Yes, you're right, this Chrysidid should be Hedychrum longicolle Abeille, even if it is easier to ID this species by direct examination of the specimen. Sometimes they can be confused with the more common H. gerstaeckeri.
I note that in Chrysis database there is no know host for H.longicolle (only: ?Halictus (Abeille, 1877)). I can tell you that I observe this Chrysididae surveying and coming inside Cerceris's burrow (gpe rybiensis: most probably C.sabulosa). This picture shows H.longicolle checking burrow, and same burrow with Cerceris head inside.
Of course Halictus could not be the host of Hedychrum spp. It was only a very old citation. On the other hand, Cerceris spp. are well known as hosts of different Hedychrum: nobile, niemelai and gerstaeckeri.
This is not a scientifical evidence, but it is surely an important observation to state that also the hosts of H. longicolle shuold be Cerceris spp.