Fix the issue of incorrect finish reason in final stream response chunk returned during tool call (#7708)

Co-authored-by: Xinyuan Tong <115166877+JustinTong0323@users.noreply.github.com>

test/srt/openai_server/function_call/test_openai_function_calling.py

@@ -159,6 +159,13 @@ class TestOpenAIServerFunctionCalling(CustomTestCase):
             "Target function name 'get_current_weather' was not found in the streaming chunks",
         )
 
+        finish_reason = chunks[-1].choices[0].finish_reason
+        self.assertEqual(
+            finish_reason,
+            "tool_calls",
+            "Final response of function calling should have finish_reason 'tool_calls'",
+        )
+
     def test_function_calling_streaming_args_parsing(self):
         """
         Test: Whether the function call arguments returned in streaming mode can be correctly concatenated into valid JSON.