fix(tools): isolate plugin runtime state

Co-authored-by: Cursor <cursoragent@cursor.com>

tests/tools/test_message_tool.py

@@ -91,6 +91,29 @@ async def test_message_tool_inherits_metadata_for_same_target() -> None:
     assert sent[0].metadata == slack_meta
 
 
+@pytest.mark.asyncio
+async def test_message_tool_clears_metadata_when_context_has_none() -> None:
+    sent: list[OutboundMessage] = []
+
+    async def _send(msg: OutboundMessage) -> None:
+        sent.append(msg)
+
+    tool = MessageTool(send_callback=_send)
+    from nanobot.agent.tools.context import RequestContext
+    tool.set_context(
+        RequestContext(
+            channel="slack",
+            chat_id="C123",
+            metadata={"slack": {"thread_ts": "111.222", "channel_type": "channel"}},
+        ),
+    )
+    tool.set_context(RequestContext(channel="slack", chat_id="C123", metadata={}))
+
+    await tool.execute(content="plain reply")
+
+    assert sent[0].metadata == {}
+
+
 @pytest.mark.asyncio
 async def test_message_tool_does_not_inherit_metadata_for_cross_target() -> None:
     sent: list[OutboundMessage] = []