Remove omitempty tag from content field in ChatCompletionStreamChoice…

[AyushSawant18588]

Describe the change
This PR removes the omitempty json tag from Content field in ChatCompletionStreamChoiceDelta for chat/completions streaming response. This is needed cause SDK which consume this response expects content field to be in the response always even when its empty/null and if this field is not present these client SDKs break. For streaming responses from VLLM the first stream chunk is:

data: {"id":"chatcmpl-25f9188232674f0bbcc81efa9cb73fc0","object":"chat.completion.chunk","created":1752484006,"model":"llama3-2","choices":[{"index":0,"delta":{"role":"assistant","content":""},"logprobs":null,"finish_reason":null}]}

So in delta only role is filled and content is empty, when this is unmarhsalled by the go-openai client it omits the content field and returns where content is missing:

data: {"id":"chatcmpl-d11a2823d2af48e09cf61b8584c5003d","object":"chat.completion.chunk","created":1752483769,"model":"llama3-2","choices":[{"index":0,"delta":{"role":"assistant"},"finish_reason":null,"content_filter_results":{"hate":{"filtered":false},"self_harm":{"filtered":false},"sexual":{"filtered":false},"violence":{"filtered":false},"jailbreak":{"filtered":false,"detected":false},"profanity":{"filtered":false,"detected":false}}}],"system_fingerprint":""}

to resolve this issue have removed omitempty tag

Provide OpenAI documentation link
https://platform.openai.com/docs/api-reference/chat_streaming/streaming
https://github.com/openai/openai-go/blob/3fb5d8d68e5e7a74fa5d666550904d26d3f34bc8/chatcompletion.go#L661
https://github.com/openai/openai-python/blob/a3315d9fcc17d7583603476f088929fb2b9e71ca/src/openai/types/chat/chat_completion_chunk.py#L60
In the official OpenAI API the content field is nullable but not omitted from the response. Also tested the official python client they do not omit the content field.

ChoiceDelta(content='', function_call=None, refusal=None, role='assistant', tool_calls=None)

Describe your solution
Removed the omitempty json tag from Content field in ChatCompletionStreamChoiceDelta for chat/completions streaming response

Tests
After updating the go-client ran chat completions streaming response and got desired output:

data: {"id":"chatcmpl-f6c03344ce134aaca4e87670c688aa74","object":"chat.completion.chunk","created":1754373814,"model":"llama3-3b","choices":[{"index":0,"delta":{"content":"","role":"assistant"},"finish_reason":null,"content_filter_results":{"hate":{"filtered":false},"self_harm":{"filtered":false},"sexual":{"filtered":false},"violence":{"filtered":false},"jailbreak":{"filtered":false,"detected":false},"profanity":{"filtered":false,"detected":false}}}],"system_fingerprint":""}

Additional context
Add any other context or screenshots or logs about your pull request here. If the pull request relates to an open issue, please link to it.

Issue: #XXXX

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.59%. Comparing base (c4273cb) to head (88aa732).

Additional details and impacted files

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##           master    #1058   +/-   ##
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  Coverage   99.59%   99.59%           
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  Files          34       34           
  Lines        2205     2205           
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  Misses          6        6           
  Partials        3        3           

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[Copilot]

Pull Request Overview

This PR removes the omitempty JSON tag from the Content field in ChatCompletionStreamChoiceDelta to ensure compatibility with OpenAI client SDKs that expect the content field to always be present in streaming responses, even when empty.

• Removes omitempty tag from the Content field in the streaming response structure
• Ensures the content field is always included in JSON serialization, matching OpenAI API behavior
• Fixes compatibility issues with client SDKs that break when the content field is missing

[Copilot]

Consider using a pointer type *string for the Content field to properly distinguish between empty string and null values, which would better match the OpenAI API specification that describes content as nullable.

Suggested change

	Content string `json:"content"`
	Content *string `json:"content"`