fix(plugin-runtime): surface detailed send failures for plugin tools

pytests/test_send_service.py

@@ -284,6 +284,22 @@ async def test_text_to_stream_returns_false_when_platform_io_fails(monkeypatch:
     assert len(fake_manager.sent_messages) == 1
 
 
+@pytest.mark.asyncio
+async def test_custom_to_stream_detailed_raises_stream_not_found(monkeypatch: pytest.MonkeyPatch) -> None:
+    monkeypatch.setattr(
+        send_service._chat_manager,
+        "get_session_by_session_id",
+        lambda stream_id: None,
+    )
+
+    with pytest.raises(send_service.SendServiceError, match="未找到聊天流: group_chat"):
+        await send_service.custom_to_stream_detailed(
+            message_type="poke",
+            content={"qq_id": "2810873701"},
+            stream_id="group_chat",
+        )
+
+
 @pytest.mark.asyncio
 async def test_private_outbound_message_preserves_bot_sender_and_receiver_user(
     monkeypatch: pytest.MonkeyPatch,