PhoenixKitAI.Provider adapter for OpenRouter's unified Images API.
POST <base_url>/images serves both generation and editing: the prompt
plus typed options (aspect_ratio, resolution, size, quality,
background, output_format, output_compression, n, seed) and,
for edits, the inputs as input_references. Every image model on the
gateway — Gemini's, OpenAI's, Seedream, FLUX, Grok Imagine, Qwen and
the rest — sits behind the same request, and GET /images/models says
per model which options it accepts (see PhoenixKitAI.Images.ImageModel).
transport: :chat keeps the older chat-completions path (image parts,
modalities: ["image","text"]) for models that only answer there.
provider_options becomes OpenRouter's provider.options object;
provider_routing (order, only, ignore, sort,
allow_fallbacks) is merged into the same provider object.