refactor(tests): optimize unit test structure

tests/tools/test_message_tool.py

@@ -0,0 +1,10 @@
+import pytest
+
+from nanobot.agent.tools.message import MessageTool
+
+
+@pytest.mark.asyncio
+async def test_message_tool_returns_error_when_no_target_context() -> None:
+    tool = MessageTool()
+    result = await tool.execute(content="test")
+    assert result == "Error: No target channel/chat specified"