Issue #537: builtin_tools/a2a_tools.py:72 returns peer-sourced text from
delegate_task() without OFFSEC-003 sanitization. Sibling regression to #491 / #492
in a different code path (google-adk delegation surface).
Fix: import sanitize_a2a_result from _sanitize_a2a and wrap all 4 peer-controlled
return sites in delegate_task() — parts[0].text path, empty-parts str(result) path,
fallback str(result) path, and the error message path.
Closes#537.
