A missing record isn't exceptional — services now return null (logged
at Info) instead of throwing, and endpoints map null to 404. Agent and
import toolsets route not-found through their existing OrNotFound
result pattern rather than a caught exception.
Not-found lookups return null/false instead of throwing NotFoundException
across all services — a missing row is expected control flow, not an
exceptional condition. NotFoundException stays for embedded precondition
checks inside mutations (missing parent, invalid foreign reference).
Guard (copied from mic-check) enforces required arguments at the top of
every service method. A ported IModelValidator<T> framework validates
every request DTO at the API layer via a new ValidationEndpointFilter,
returning a 400 with field-level messages; services re-run the same
validator and throw for direct callers that bypass the API.
Endpoints translate null/false into 404 via a new ToApiResult() helper.
The agent toolset boundary translates the same nullable/bool results
into the tool-error text the model already expected.
James Wampler