AddChunk panics with index out of range when tool_calls is empty

[kklinan]

Description:
When calling AddChunk, if the tool_calls field in the delta is an empty array, a panic occurs due to an out-of-range array access in https://github.com/openai/openai-[go/blob/main/streamaccumulator.go#L172](https://www.golinks.io/blob/main/streamaccumulator.go#L172?trackSource=github)

panic: runtime error: index out of range [0] with length 0

goroutine 59 [running]:
github.com/openai/openai-go.(*chatCompletionResponseState).update(_, {{0xc00053a277, 0x29}, {0xc00034f688, 0x1, 0x1}, 0x68673945, {0xc00053a2e1, 0xa}, {0xc00053a2ac, ...}, ...})
	/Users/admin/Workspaces/Gomod/**/vendor/github.com/openai/openai-go/streamaccumulator.go:172 +0x1e5
github.com/openai/openai-go.(*ChatCompletionAccumulator).AddChunk(_, {{0xc00053a277, 0x29}, {0xc00034f688, 0x1, 0x1}, 0x68673945, {0xc00053a2e1, 0xa}, {0xc00053a2ac, ...}, ...})

Cause:
The panic is caused by delta.JSON.ToolCalls.Valid() returning true even when tool_calls is an empty array. The code then proceeds to access elements without checking the array length, assuming at least one item exists.

Reproduction:
This JSON response chunk triggers the issue:

{
  "id": "chatcmpl-20299db6f3b64cc982ca667550aed702",
  "choices": [
    {
      "delta": {
        "content": "",
        "function_call": {
          "arguments": "",
          "name": ""
        },
        "refusal": "",
        "role": "assistant",
        "tool_calls": null
      },
      "finish_reason": "",
      "index": 0,
      "logprobs": {
        "content": null,
        "refusal": null
      }
    }
  ],
  "created": 1751595333,
  "model": "qwen3-235b",
  "object": "chat.completion.chunk",
  "usage": {
    "completion_tokens": 0,
    "prompt_tokens": 0,
    "total_tokens": 0,
    "completion_tokens_details": {
      "accepted_prediction_tokens": 0,
      "audio_tokens": 0,
      "reasoning_tokens": 0,
      "rejected_prediction_tokens": 0
    },
    "prompt_tokens_details": {
      "audio_tokens": 0,
      "cached_tokens": 0
    }
  }
}

Expected Behavior:
The code should safely handle cases where tool_calls is null or empty, avoiding index access unless the array has valid entries.

Proposed Fix:
Add a length check before accessing tool_calls in update().