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/watcher/diff/openai_compat.go

@@ -153,7 +153,7 @@ func openAICompatSignature(entry config.OpenAICompatibility) string {
 		if name == "" && alias == "" {
 			continue
 		}
-		models = append(models, strings.ToLower(name)+"|"+strings.ToLower(alias))
+		models = append(models, strings.ToLower(name)+"|"+strings.ToLower(alias)+"|"+fmt.Sprintf("image=%t", model.Image))
 	}
 	if len(models) > 0 {
 		sort.Strings(models)