feat(fm): answer GetFederationDetails as the partner

Two FMs couldn't read each other's federation details: FM served only
DELETE on /{federationContextId}/partner, so a partner's GET got a 405
back, which the OEG -> FM -> FM e2e run caught.

FM now returns its network codes and platformCaps. Offered zones and the
edge-discovery and LCM endpoints are left out rather than sent empty,
because FM has no zone catalogue or endpoint config yet and OPG.04 marks
none of them nullable.