capture stderr for python tool with uv

Signed-off-by: wuhang <wuhang6@huawei.com>

Author: wuhang2014

gpt_oss/tools/python_docker/docker_tool.py

@@ -78,7 +78,9 @@ def call_python_script_with_uv(script: str) -> str:
         exec_result = subprocess.run(
             ["uv", "run", "--no-project", "python", script_path],
             capture_output=True)
-        return exec_result.stdout.decode("utf-8")
+        if exec_result.returncode == 0:
+            return exec_result.stdout.decode("utf-8")
+        return exec_result.stderr.decode("utf-8")
 
 
 class PythonTool(Tool):