Continues the OSS-shape refactor. After iters 4a-4d (rbac, delegation,
memory, messaging) the only behavior left in ``a2a_tools.py`` was
``report_activity`` plus three thin inbox-tool wrappers and the
``_enrich_inbound_for_agent`` helper. This iter extracts the inbox
slice to ``a2a_tools_inbox.py`` so the kitchen-sink module shrinks
from 280 LOC to ~165 LOC of imports + report_activity + back-compat
re-export blocks.
Extracted symbols:
- ``_INBOX_NOT_ENABLED_MSG`` (sentinel)
- ``_enrich_inbound_for_agent`` (poll-path peer enrichment helper)
- ``tool_inbox_peek``
- ``tool_inbox_pop``
- ``tool_wait_for_message``
Re-exports (`from a2a_tools_inbox import …`) preserve the public
``a2a_tools.tool_inbox_*`` surface so existing tests + call sites
continue to resolve unchanged.
New tests in test_a2a_tools_inbox_split.py:
1. **Drift gate (5)** — every previously-public symbol on a2a_tools
is the EXACT same object as a2a_tools_inbox.foo (`is`, not `==`),
catches a future "wrap with logging" refactor that silently loses
existing test coverage.
2. **Import contract (1)** — a2a_tools_inbox does NOT eagerly import
a2a_tools at module load. Pins the layered architecture: the
extracted slice depends on ``inbox`` + a lazy ``a2a_client``
import, never on the kitchen-sink that re-exports it.
3. **_enrich_inbound_for_agent branches (5)** — peer_id-empty
(canvas_user) returns dict unchanged; missing peer_id key same;
a2a_client unavailable (test harness, partial install) degrades
gracefully with a bare envelope; registry hit populates
peer_name + peer_role + agent_card_url; registry miss still
surfaces agent_card_url (constructable from peer_id alone).
The full timeout-clamp / validation / JSON-shape behavior matrix for
the three wrappers stays in test_a2a_tools_inbox_wrappers.py — those
tests pass identically against both the alias and the underlying impl.
Wiring updates:
- ``scripts/build_runtime_package.py``: add ``a2a_tools_inbox`` to
``TOP_LEVEL_MODULES`` so it ships in the runtime wheel and the
drift gate doesn't fail the next publish.
- ``.github/workflows/ci.yml``: add ``a2a_tools_inbox.py`` to
``CRITICAL_FILES`` so the 75% MCP/inbox/auth per-file floor
applies — this is now where the inbox-delivery code actually
lives.
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