Added support for images (#609)

app/MindWork AI Studio/Provider/OpenAI/SubContentImageUrlNested.cs

@@ -0,0 +1,18 @@
+namespace AIStudio.Provider.OpenAI;
+
+/// <summary>
+/// Image sub-content for multimodal messages using nested URL format.
+/// </summary>
+/// <remarks>
+/// This record is used when the provider expects the format:
+/// <code>
+/// { "type": "image_url", "image_url": { "url": "data:image/jpeg;base64,..." } }
+/// </code>
+/// Used by LM Studio, VLLM, and other OpenAI-compatible providers.
+/// </remarks>
+public record SubContentImageUrlNested(SubContentType Type, ISubContentImageUrl ImageUrl) : ISubContent
+{
+    public SubContentImageUrlNested() : this(SubContentType.IMAGE_URL, new SubContentImageUrlData())
+    {
+    }
+}