refactor: simplify cross-session messaging

tests/tools/test_message_tool.py

@@ -15,21 +15,6 @@ async def test_message_tool_returns_error_when_no_target_context() -> None:
     assert result == "Error: No target channel/chat specified"
 
 
-@pytest.mark.asyncio
-async def test_message_tool_preserves_legacy_positional_channel_arguments() -> None:
-    sent: list[OutboundMessage] = []
-
-    async def send(message: OutboundMessage) -> None:
-        sent.append(message)
-
-    tool = MessageTool(send_callback=send)
-    await tool.execute("hello", "telegram", "chat-1")
-
-    assert [(message.channel, message.chat_id, message.content) for message in sent] == [
-        ("telegram", "chat-1", "hello")
-    ]
-
-
 @pytest.mark.asyncio
 @pytest.mark.parametrize(
     "bad",