fix: omit null delta fields in streaming chat completions (issue #2082)

[Heidar-An]

Motivation

Streaming /v1/chat/completions responses emitted null for tool_calls,
function_call, name, and tool_call_id in every delta chunk. The OpenAI
streaming spec marks these fields as non-nullable — they must either carry a
real value or be absent entirely. Spec-correct clients doing
delta.get("tool_calls", []) receive None and crash with 'NoneType' object is
not iterable.

Root cause: the streaming serialisation path called model_dump_json() without
exclude_none=True, while the request-parsing path already used it correctly.
Three call sites in chat_completions.py and two in responses.py were affected.

Testing

Before — every delta carries explicit nulls:

$ curl -sN -X POST http://localhost:52415/v1/chat/completions
-H 'Content-Type: application/json'
-d '{"model":"mlx-community/Qwen3.5-2B-MLX-8bit","messages":[{"role":"user","
content":"hi"}],"max_tokens":3,"stream":true}'
| grep "^data: "
data: {"id":"7c4dae10-...","choices":[{"index":0,"delta":{"role":"assistant","c
ontent":null,"reasoning_content":"Okay","name":null,"tool_calls":null,"tool_cal
l_id":null,"function_call":null},"logprobs":null,"finish_reason":null,"usage":n
ull}],"usage":null,"service_tier":null}
data: {"id":"7c4dae10-...","choices":[{"index":0,"delta":{"role":"assistant","c
ontent":null,"reasoning_content":",","name":null,"tool_calls":null,"tool_call_i
d":null,"function_call":null},"logprobs":null,"finish_reason":null,"usage":null
}],"usage":null,"service_tier":null}
data: {"id":"7c4dae10-...","choices":[{"index":0,"delta":{"role":"assistant","c
ontent":" the","reasoning_content":null,"name":null,"tool_calls":null,"tool_cal
l_id":null,"function_call":null},"logprobs":null,"finish_reason":"length","usag
e":{"prompt_tokens":11,...}}],"usage":null,"service_tier":null}
data: [DONE]

After — only populated fields are emitted:
data: {"id":"demo","object":"chat.completion","created":...,"model":"mlx-commun
ity/Qwen3.5-2B-MLX-8bit","choices":[{"index":0,"delta":{"role":"assistant","rea
soning_content":"Okay"}}]}
data: {"id":"demo","object":"chat.completion","created":...,"model":"mlx-commun
ity/Qwen3.5-2B-MLX-8bit","choices":[{"index":0,"delta":{"role":"assistant","rea
soning_content":","}}]}
data: {"id":"demo","object":"chat.completion","created":...,"model":"mlx-commun
ity/Qwen3.5-2B-MLX-8bit","choices":[{"index":0,"delta":{"role":"assistant","con
tent":" the"},"finish_reason":"length"}],"usage":{"prompt_tokens":11,"completio
n_tokens":3,"total_tokens":14,...}}
data: [DONE]

[Evanev7]

not certain this is a complete solution, but it certainly doesn't hurt. 3/3!