add video url support to chat.completions

[orangetin]

• Add VIDEO_URL type to chat completion message content types
• Implement ChatCompletionMessageContentVideoURL model for video URL data
• Add video_url field to ChatCompletionMessageContent class
• Add unit test to verify video URL message handling
• Update README with multi-modal content documentation including:
  • Text-only messages
  • Image URL messages
  • Video URL messages
  • Clear explanation of content type usage

Example usage:

client.chat.completions.create(
    messages=[{
        "role": "user",
        "content": [
            {"type": "text", "text": "What's in this video?"},
            {"type": "video_url", "video_url": {"url": "https://example.com/video.mp4"}},
        ],
    }],
    model="your_model"
)

[ConnorLi96]

lgtm