[BUG] ChatCompletionsToolCall missing index field

[anuraaga]

Describe the bug

OpenAI response for a tool call contains a field index. This is particularly important for streaming tool call requests because the tool calls are returned individually within chunks. Without the index, it is not clear which tool call a particular chunk is referring to. While currently it seems OpenAI only returns id in the first chunk of a tool call and that could be used as a delimiter, I think that behavior could change in the future and isn't intended to be how we delimit tool calls in streaming.

JSON response:

{
  "id" : "chatcmpl-AiWm43aBO7oeXWZ3bOGNh48zGSfDN",
  "object" : "chat.completion.chunk",
  "created" : 1735175780,
  "model" : "gpt-4o-mini-2024-07-18",
  "system_fingerprint" : "fp_0aa8d3e20b",
  "choices" : [ {
    "index" : 0,
    "delta" : {
      "tool_calls" : [ {
        "index" : 0,
        "id" : "call_c5QfAjfoKgdc8TLCZwCLqe75",
        "type" : "function",
        "function" : {
          "name" : "get_weather",
          "arguments" : ""
        }
      } ]
    },
    "logprobs" : null,
    "finish_reason" : null
  } ]
}

index field not parsed:
https://github.com/Azure/azure-sdk-for-java/blob/main/sdk/openai/azure-ai-openai/src/main/java/com/azure/ai/openai/models/ChatCompletionsFunctionToolCall.java#L71

Exception or Stack Trace
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To Reproduce

Execute a streaming parallel tool call request. OpenAI docs only have one for streaming tool calls, and parallel tool calls, not parallel streaming tool calls but they can be combined to exercise the functionality.

https://platform.openai.com/docs/guides/function-calling#advanced-usage

Code Snippet
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    private static class StringField {
        private String type = "string";

        private String description;

        public StringField(String description) {
            this.description = description;
        }
    }

    private static class GetWeatherFunctionProperties {
        StringField
                location = new StringField("The location to get the current temperature for");
    }

    private static class GetWeatherFunctionParameterDefinition {
        private String type = "object";

        private List<String> required = List.of("location");

        private boolean additionalProperties = false;

        private GetWeatherFunctionProperties
                properties = new GetWeatherFunctionProperties();
    }

    void streamParallelToolsAndCaptureMessageContent(boolean useResponseApi) {
        FunctionDefinition functionDefinition = new FunctionDefinition("get_weather");
        GetWeatherFunctionParameterDefinition
                parameters = new GetWeatherFunctionParameterDefinition();
        functionDefinition.setParameters(BinaryData.fromObject(parameters));

        List<ChatRequestMessage> chatMessages = new ArrayList<>();
        chatMessages.add(new ChatRequestSystemMessage("You are a helpful assistant providing weather updates."));
        chatMessages.add(new ChatRequestUserMessage("What is the weather in New York City and London?"));

        ChatCompletionsOptions options = new ChatCompletionsOptions(chatMessages)
                .setTools(List.of(new ChatCompletionsFunctionToolDefinition(functionDefinition)));


        IterableStream<ChatCompletions> chatCompletionsStream = openai.client.getChatCompletionsStream(TEST_CHAT_MODEL, options);

Expected behavior
A clear and concise description of what you expected to happen.

ChatCompletions.choices.delta.toolCalls have index populated

Issue is obvious from ChatCompletionsFunctionToolCall code so will leave out remaining.

Screenshots
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Setup (please complete the following information):

• OS: [e.g. iOS]
• IDE: [e.g. IntelliJ]
• Library/Libraries: [e.g. com.azure:azure-core:1.16.0 (groupId:artifactId:version)]
• Java version: [e.g. 8]
• App Server/Environment: [e.g. Tomcat, WildFly, Azure Function, Apache Spark, Databricks, IDE plugin or anything special]
• Frameworks: [e.g. Spring Boot, Micronaut, Quarkus, etc]

If you suspect a dependency version mismatch (e.g. you see NoClassDefFoundError, NoSuchMethodError or similar), please check out Troubleshoot dependency version conflict article first. If it doesn't provide solution for the problem, please provide:

• verbose dependency tree (mvn dependency:tree -Dverbose)
• exception message, full stack trace, and any available logs

Additional context
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