feat(api): add OpenAI compatibility for image models

- Introduced OpenAI-compatible image model support in the API, enabling integration through image generation and editing endpoints.
- Added registry type for OpenAIImageModelType to classify and validate compatibility.
- Implemented request handling for OpenAI-compatible image models, including JSON and multipart formats.
- Enhanced executor methods to support OpenAI-compatible image streaming and non-streaming requests.
- Included tests to validate model registration, streaming behavior, and multipart payload formatting.

internal/registry/model_registry.go

@@ -15,6 +15,9 @@ import (
 	log "github.com/sirupsen/logrus"
 )
 
+// OpenAIImageModelType marks models that are callable through OpenAI-compatible image endpoints.
+const OpenAIImageModelType = "openai-image"
+
 // ModelInfo represents information about an available model
 type ModelInfo struct {
 	// ID is the unique identifier for the model