Assistant raise exceptions when OpenAI tool_calls is nil

[littlechu]

Describe the bug
Seem like in OpenAIMessage can pass in tool_calls = nil. which causes exceptions being throw

To Reproduce
Create OpenAI assistant agent with something simple, like the following
assistant = Langchain::Assistant.new(
llm: openai,
instructions: "You're a helpful AI assistant",
tools: [Langchain::Tool::FileSystem.new, Langchain::Tool::Calculator.new]
)
assistant.add_message_and_run!(content: 'how many directory are there under /Home')
this will result in something like this
raise ArgumentError, "Tool calls must be an array of hashes" unless tool_calls.is_a?(Array) && tool_calls.all? { |tool_call| tool_call.is_a?(Hash) }

Expected behavior
I expect no exception is being raise.

• OS: [e.g. OS X, Linux, Ubuntu, Windows]
• Ruby version 3.4.2
• Langchain.rb version 0.19.5

Additional context
By patching the following two places, the code seem to be working
lib/langchain/assistant/messages/openai_message.rb:31 add tool_calls ||=[]
lib/langchain/assistant.rb:312 change response.tool_calls.any? to response.tool_calls&.any?

Seem like it's possible for the repose body to contain "tool_calls":null from openai gems

I, [2025-06-30T14:39:13.533775 #94841] INFO -- [Langchain.rb]: response: {"id":"chatcmpl-BoGVVtq92F0jUwOIs6CU0qs65xbql","created":1751319553,"model":"gpt-4o-mini-2024-07-18","object":"chat.completion","system_fingerprint":"fp_efad92c60b","choices":[{"finish_reason":"stop","index":0,"message":{"content":"There are 2 entries in the /Home directory: . (current directory) and .. (parent directory). It appears that there are no additional directories under /Home.","role":"assistant","tool_calls":null,"function_call":null,"annotations":[]},