test(discord): update tests for bot-to-bot fix (#3217)

The old test `test_on_message_ignores_bot_messages` asserted the
previous (incorrect) contract that ALL bot-authored messages are
dropped. With #3217 only self-loops are dropped, so this test was
replaced with three more precise tests:

- test_on_message_ignores_self_messages: verifies self-loop guard
  (author_id == _bot_user_id is dropped)
- test_on_message_accepts_messages_from_other_bots: new test for
  the fix itself — other bots' messages flow through
- test_on_message_stops_typing_on_handle_exception: preserves the
  typing cleanup assertion from the original test

Net result: +1 behavior tested, same behaviors retained.

Co-authored with Claude Opus 4.7

tests/channels/test_discord_channel.py

@@ -273,17 +273,41 @@ async def test_stop_is_safe_after_partial_start(monkeypatch) -> None:
 
 
 @pytest.mark.asyncio
-async def test_on_message_ignores_bot_messages() -> None:
-    # Incoming bot-authored messages must be ignored to prevent feedback loops.
+async def test_on_message_ignores_self_messages() -> None:
+    # Self-loop guard: messages from this bot's own account must be dropped (#3217).
     channel = DiscordChannel(DiscordConfig(enabled=True, allow_from=["*"]), MessageBus())
+    channel._bot_user_id = "999"  # simulate bot identity populated in on_ready()
     handled: list[dict] = []
     channel._handle_message = lambda **kwargs: handled.append(kwargs)  # type: ignore[method-assign]
 
-    await channel._on_message(_make_message(author_bot=True))
+    await channel._on_message(_make_message(author_id=999, author_bot=True))
 
     assert handled == []
 
+
+@pytest.mark.asyncio
+async def test_on_message_accepts_messages_from_other_bots() -> None:
+    # Multi-agent setups: messages from OTHER bots must be processed, not dropped (#3217).
+    channel = DiscordChannel(DiscordConfig(enabled=True, allow_from=["*"]), MessageBus())
+    channel._bot_user_id = "999"
+    handled: list[dict] = []
+
+    async def capture_handle(**kwargs) -> None:
+        handled.append(kwargs)
+
+    channel._handle_message = capture_handle  # type: ignore[method-assign]
+
+    await channel._on_message(_make_message(author_id=123, author_bot=True))
+
+    assert len(handled) == 1
+    assert handled[0]["sender_id"] == "123"
+
+
+@pytest.mark.asyncio
+async def test_on_message_stops_typing_on_handle_exception() -> None:
     # If inbound handling raises, typing should be stopped for that channel.
+    channel = DiscordChannel(DiscordConfig(enabled=True, allow_from=["*"]), MessageBus())
+
     async def fail_handle(**kwargs) -> None:
         raise RuntimeError("boom")