Replace the undefined tools variable with tool_schema parameter in ToolUseAgent class (#5684)

Replace the undefined `tools` variable with `tool_schema` parameter

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## Why are these changes needed?
This change keeps the documentation up to date :
https://microsoft.github.io/autogen/stable//user-guide/core-user-guide/components/tools.html

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## Related issue number

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## Checks

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<https://microsoft.github.io/autogen/>. See
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python/packages/autogen-core/docs/src/user-guide/core-user-guide/components/tools.ipynb

@@ -300,7 +300,7 @@
     "        super().__init__(\"An agent with tools\")\n",
     "        self._system_messages: List[LLMMessage] = [SystemMessage(content=\"You are a helpful AI assistant.\")]\n",
     "        self._model_client = model_client\n",
-    "        self._tools = tools\n",
+    "        self._tools = tool_schema\n",
     "\n",
     "    @message_handler\n",
     "    async def handle_user_message(self, message: Message, ctx: MessageContext) -> Message:\n",