Api Testsets Create 3

Default POST handler with automatic organization injection.

For CREATE operations:

• Superadmins can specify organization_id in request body (API key only)
• Regular users always use their own organization

Override this method for:

• POST-for-filtering pattern (delegate to self.get())
• Custom pre-create validation

Note: inject_target_organization is a DEPRECATED request.data-mutating shim kept for backward-compat during the DEV-9410 migration. The blessed path stamps org via get_create_save_kwargs → perform_create → a read_only serializer field. The shim (and these overrides' reliance on it) is removed in C18 (DEV-9430) once every view's org field is read_only.