Refuse a transaction on any simulator 400 once auth-server reports rejection codes everywhere

Follow-up to #489 and decentraland/auth-server#146.

`fetchSimulation` in `src/components/Pages/RequestPage/RequestPage.tsx` refuses a transaction review on a simulator 400 only when the server accounts for it as `invalid_request`, and degrades a 400 with no code to the unavailable preview plus acknowledgment. That asymmetry exists so a client deployed ahead of the server never refuses a legitimate transaction on a provider-side 400 (a server from before the codes sends none). Signatures already refuse on any 400 that is not `upstream_rejected`.

Once every auth-server deployment answers `POST /simulations` with a `code`, tighten the transaction rule to match the signature rule: refuse unless the code is `upstream_rejected`. Update the page spec "answers a 400 without saying why, as a server from before the codes does" accordingly.