Orgs Feature Parity: Cross-org validation for Offline Transfer

Problem

Import::Offline::Imports::CreateService (called from API::OfflineTransfers) has the same cross-organization gap as Direct Transfer: a user scoped to org A can start an offline import that lands in a group belonging to org B. Same shape as #595674 (closed), different service.

Proposed fix

Mirror the isolated?-gated check from #595674 (closed) / !236603 (merged): reject when either the request org or the resolved destination org is marked isolated, no-op otherwise.

If the gate ends up reused beyond these two services, consider extracting a shared helper (e.g. Import::Framework::OrganizationValidator) — not required for this issue.

Refs

Follow-up to #595674 (closed). Part of &21584 (closed). Related: &17388, &19415.