Issue: _delegate_sync_via_polling (RFC #2829 PR-5 sync path) returned
unsanitized response_preview and error_detail fields to the agent context.
A malicious peer could inject trust-boundary markers to break the boundary
established by the main sanitization layer.
Changes:
- a2a_tools_delegation.py: sanitize response_preview before returning on
completed; sanitize error_detail/summary before wrapping in _A2A_ERROR_PREFIX
- test_a2a_tools_delegation.py: TestPollingPathSanitization covers both paths
Companion to PR #382 (runtime/offsec-003-executor-sanitize) which covers
the async heartbeat path in executor_helpers.read_delegation_results.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>