refactor: use module-level constant for OS name as suggested in PR review

Author: miaoxutao123

astrbot/core/computer/tools/python.py

@@ -11,6 +11,8 @@
 from astrbot.core.computer.tools.permissions import check_admin_permission
 from astrbot.core.message.message_event_result import MessageChain
 
+_OS_NAME = platform.system()
+
 param_schema = {
     "type": "object",
     "properties": {
@@ -62,9 +64,7 @@ async def handle_result(result: dict, event: AstrMessageEvent) -> ToolExecResult
 @dataclass
 class PythonTool(FunctionTool):
     name: str = "astrbot_execute_ipython"
-    description: str = (
-        f"Run codes in an IPython shell. Current OS: {platform.system()}."
-    )
+    description: str = f"Run codes in an IPython shell. Current OS: {_OS_NAME}."
     parameters: dict = field(default_factory=lambda: param_schema)
 
     async def call(
@@ -86,7 +86,10 @@ async def call(
 @dataclass
 class LocalPythonTool(FunctionTool):
     name: str = "astrbot_execute_python"
-    description: str = f"Execute codes in a Python environment. Current OS: {platform.system()}. Use system-compatible commands."
+    description: str = (
+        f"Execute codes in a Python environment. Current OS: {_OS_NAME}. "
+        "Use system-compatible commands."
+    )
 
     parameters: dict = field(default_factory=lambda: param_schema)