Fix/route agency db error

[FLASH2332]

Problem:
In routeHandler, the error returned by GetAgency was silently discarded:
goagency, err := api.GtfsManager.GtfsDB.Queries.GetAgency(ctx, agencyID)

if err == nil { // agency added to references } // any error is silently dropped 

If this DB call failed for any reason — including context.Canceled or context.DeadlineExceeded — the handler would skip the agency reference and return a successful response with incomplete data.

Fix :
sql.ErrNoRows → agency reference is optional, skip silently and continue
Any other error → route through serverErrorResponse, which correctly handles context cancellation and deadline exceeded

Changes :
internal/restapi/route_handler.go — added explicit error handling for GetAgency

Closes #792