Fix ChatCompletionStreamOptions serialization for OpenAI-compatible providers

[ewgenius]

Problem

When include_obfuscation is None, it is serialized as "include_obfuscation": null instead of being omitted from the JSON payload. Some OpenAI-compatible providers (e.g., NVIDIA NIM) reject requests containing unknown fields, causing errors like:

failed deserialization of: {"object":"error","message":"[{'type': 'extra_forbidden', 'loc': ('body', 'stream_options', 'include_obfuscation'), 'msg': 'Extra inputs are not permitted', 'input': None}]","type":"BadRequestError","param":null,"code":400}

Added #[serde(skip_serializing_if = "Option::is_none")] to both fields in ChatCompletionStreamOptions, so None values are omitted from serialization rather than being sent as null

[64bit]

Usually for this test is not required, thanks for the PR!