Fourth slice of the a2a_tools.py split (stacked on iter 4c). Owns the
four human-and-peer messaging MCP tools + the chat-upload helper:
* _upload_chat_files — stage local paths to /chat/uploads
* tool_send_message_to_user — push canvas-chat via /notify
* tool_list_peers — discover peers across registered workspaces
* tool_get_workspace_info — JSON-encode workspace info
* tool_chat_history — fetch prior conversation rows with a peer
a2a_tools.py shrinks from 508 → 213 LOC (−295). The remaining 213
is just report_activity + back-compat re-exports. Inbox tools
(tool_inbox_peek/pop/wait_for_message) deferred to iter 4e.
Layered architecture: messaging depends on a2a_tools_rbac (iter 4a),
a2a_client, platform_auth — NOT on kitchen-sink a2a_tools. An
import-contract test pins this so future refactors that add
`from a2a_tools import …` fail in CI.
Tests:
* 28 patch sites in TestToolSendMessageToUser + TestToolListPeers +
TestToolGetWorkspaceInfo + TestChatHistory retargeted from
`a2a_tools.{httpx, get_peers_*, get_workspace_info,
_upload_chat_files, _peer_*, list_registered_workspaces}` to
`a2a_tools_messaging.…` because the call sites moved.
* test_a2a_tools_messaging.py adds 7 new tests:
- 5 alias drift gates
- 2 import-contract tests (no top-level a2a_tools dep + a2a_tools
surfaces every messaging symbol)
137 tests total in the a2a_tools suite, all green.
Refs RFC #2873.
