Adds _sanitize_a2a.py (from PR #346) and integrates sanitize_a2a_result()
into read_delegation_results() so peer-supplied summary and response_preview
fields are escaped before being injected into the agent prompt.
Output is wrapped in [A2A_RESULT_FROM_PEER]...[/A2A_RESULT_FROM_PEER]
boundary markers so content after the block is clearly not from a peer.
Fixes:
- test_a2a_executor.py: correct mock patch path to executor_helpers
- test_executor_helpers.py: fix boundary-injection test assertion to match
_strip_closed_blocks behaviour (closes marker, removes following text)
Follow-up to PR #346 (OFFSEC-003 boundary escape) which noted
"read_delegation_results() path still needs sanitization" as a gap.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
