Defines how the solver should handle the request. In all modes but VALIDATE_ONLY, if the request is invalid, you will receive an INVALID_REQUEST error with the first error encountered.

DEFAULT_SOLVE Solve the model.
VALIDATE_ONLY Only validates the model without solving it: populates as many OptimizeToursResponse.validation_errors as possible.

Only populates OptimizeToursResponse.skipped_shipments, and doesn't actually solve the rest of the request (status and routes are unset in the response).

IMPORTANT: not all infeasible shipments are returned here, but only the ones that we detect as infeasible as a preprocessing.