refine tool提示语为更简洁风格，补充last_buffer_char属性及非VERBOSE模式流式输出换行逻辑，新增工具流式分隔符单元测试

tests/test_agent_tool_streaming.py

@@ -0,0 +1,53 @@
+import asyncio
+import unittest
+from unittest.mock import patch
+
+from app.agent.callback import StreamingHandler
+from app.agent.tools.base import MoviePilotTool
+from app.core.config import settings
+
+
+class DummyTool(MoviePilotTool):
+    name: str = "dummy_tool"
+    description: str = "Dummy tool for streaming tests."
+
+    async def run(self, **kwargs) -> str:
+        return "ok"
+
+
+class TestAgentToolStreaming(unittest.TestCase):
+    async def _run_tool(self, initial_buffer: str) -> tuple[str, str]:
+        tool = DummyTool(session_id="session-1", user_id="10001")
+        handler = StreamingHandler()
+        await handler.start_streaming()
+        if initial_buffer:
+            handler.emit(initial_buffer)
+        tool.set_stream_handler(handler)
+
+        with patch.object(settings, "AI_AGENT_VERBOSE", False):
+            result = await tool._arun(explanation="run test tool")
+
+        buffered_message = await handler.take()
+        return result, buffered_message
+
+    def test_non_verbose_tool_call_appends_newline_separator(self):
+        result, buffered_message = asyncio.run(self._run_tool("prefix"))
+
+        self.assertEqual(result, "ok")
+        self.assertEqual(buffered_message, "prefix\n")
+
+    def test_non_verbose_tool_call_does_not_duplicate_newline(self):
+        result, buffered_message = asyncio.run(self._run_tool("prefix\n"))
+
+        self.assertEqual(result, "ok")
+        self.assertEqual(buffered_message, "prefix\n")
+
+    def test_non_verbose_tool_call_keeps_empty_buffer_unchanged(self):
+        result, buffered_message = asyncio.run(self._run_tool(""))
+
+        self.assertEqual(result, "ok")
+        self.assertEqual(buffered_message, "")
+
+
+if __name__ == "__main__":
+    unittest.main()