REGRESSION: Staging commit 8e94c178 (PR #390) added sanitize_a2a_result
calls to _delegate_sync_via_polling but did NOT add the import. Any
delegation completing via the polling path raises NameError at runtime.
One-line fix: add `from _sanitize_a2a import sanitize_a2a_result`.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>