Opportunistically resolves a pasted SharePoint web URL into Microsoft Graph
identifiers, routing to the native connection for the URL's Entra tenant.

Tenant, host, site and the routed connection_id are always returned; the
drive, library, folder path and composed external_source_id are best-effort
and may be null (e.g. a SitePages URL). When no connection exists for the
tenant, returns a connection_required_for_tenant error carrying the
sharepoint_host and tenant_id so the frontend can launch a tenant-pinned
connect flow.