misc/py-haystack_ai: Update to 2.18.1

• Enhancement Notes:
  • Added tools to agent run parameters to enhance the agent's flexibility. Users can now choose a subset of tools for the agent at runtime by providing a list of tool names, or supply an entirely new set by passing Tool objects or a Toolset.

• Bug Fixes
  • Fix Agent run_async method to correctly handle async streaming callbacks. This previously triggered errors due to a bug.
  • Prevent duplication of the last assistant message in the chat history when initializing from an AgentSnapshot.
  • We were setting response_format to None in OpenAIChatGenerator by default which doesn't follow the API spec. We now omit the variable if response_format is not passed by the user.

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