Audit every message across nuncio.proto for the id-naming convention
(an entity's own identifier is `id`; a reference to another entity is
`<noun>_id`) and fix the outliers: RemoveAccountRequest, TestAccountConnectionRequest,
DeleteRuleRequest, and UpdateRuleRequest carried a bare `id` referencing an
account or rule rather than naming it; MessageSearchHit did the same for
the message it matched. AddAccountResponse now returns the persisted
AccountConfig (never the password), matching how CreateRule/CreateContact
return their created object instead of a bare id.
Document Message.id (and the other entity ids) as an opaque,
server-assigned token clients must never parse or construct, reflecting
the surrogate id introduced for messages.
Regenerates the nuncio.v1 descriptor golden for the renamed/changed fields.
Refs #276
James Maes