Clients - Platform Import

Import clients from third-party platforms (e.g., Xero, QuickBooks) or via direct import with comprehensive data. Includes direct import (create client with all related data in one request), listing available clients from connected platforms, and importing selected or all clients with optional Companies House enrichment. Uses a sync-when-fast pattern for platform imports (waits up to 10 seconds, returns 201 if completed, 202 if still processing).