OpenRouter supports image editing through its chat completions endpoint
for models like google/gemini-2.5-flash-image, but LiteLLM raised
ValueError("image edit is not supported for openrouter") because
OpenRouter was not registered as an image edit provider.
Add OpenRouterImageEditConfig that routes image edit requests through
the chat completions endpoint with the source image as a base64 data
URL in the message content array.
Fixes https://github.com/BerriAI/litellm/issues/22305