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/config/config.go

@@ -585,6 +585,9 @@ type OpenAICompatibilityModel struct {
 	// Alias is the model name alias that clients will use to reference this model.
 	Alias string `yaml:"alias" json:"alias"`
 
+	// Image marks this model as callable through /v1/images/generations and /v1/images/edits.
+	Image bool `yaml:"image,omitempty" json:"image,omitempty"`
+
 	// Thinking configures the thinking/reasoning capability for this model.
 	// If nil, the model defaults to level-based reasoning with levels ["low", "medium", "high"].
 	Thinking *registry.ThinkingSupport `yaml:"thinking,omitempty" json:"thinking,omitempty"`