Second slice of the a2a_tools.py split (stacked on iter 4a). Owns the
three delegation MCP tools + the RFC #2829 PR-5 sync-via-polling
helper they share:
* tool_delegate_task — synchronous delegation
* tool_delegate_task_async — fire-and-forget
* tool_check_task_status — poll the platform's /delegations log
* _delegate_sync_via_polling — durable async + poll for terminal status
* _SYNC_POLL_INTERVAL_S / _SYNC_POLL_BUDGET_S constants
a2a_tools.py shrinks from 915 → 609 LOC (−306). Stacked on iter 4a's
RBAC extraction; uses `from a2a_tools_rbac import auth_headers_for_heartbeat`
as its auth-header source.
The lazy `from a2a_tools import report_activity` inside tool_delegate_task
breaks the circular-import cycle (a2a_tools imports the delegation
re-exports at module-load; delegation handler needs report_activity at
CALL time). A dedicated test pins this contract.
Tests:
* 77 existing test_a2a_tools_impl.py tests pass after retargeting
20 patch sites in TestToolDelegateTask + TestToolDelegateTaskAsync +
TestToolCheckTaskStatus from `a2a_tools.foo` to
`a2a_tools_delegation.foo` (foo ∈ {discover_peer, send_a2a_message,
httpx.AsyncClient}). The patches need to target the new module
because that's where the call sites live now.
* test_a2a_tools_delegation.py adds 8 new tests:
- 6 alias drift gates (`a2a_tools.tool_delegate_task is …`)
- 2 import-contract tests (no top-level circular dep + a2a_tools
surfaces every delegation symbol)
- 1 sync-poll budget invariant
113 tests total (77 impl + 28 rbac + 8 delegation), all green.
Refs RFC #2873.
