feat: add auto model resolution and model creation timestamp tracking

- Add 'created' field to model registry for tracking model creation time
- Implement GetFirstAvailableModel() to find the first available model by newest creation timestamp
- Add ResolveAutoModel() utility function to resolve "auto" model name to actual available model
- Update request handler to resolve "auto" model before processing requests
- Ensures automatic model selection when "auto" is specified as model name

This enables dynamic model selection based on availability and creation time, improving the user experience when no specific model is requested.

internal/util/provider.go

@@ -9,6 +9,7 @@ import (
 
 	"github.com/router-for-me/CLIProxyAPI/v6/internal/config"
 	"github.com/router-for-me/CLIProxyAPI/v6/internal/registry"
+	log "github.com/sirupsen/logrus"
 )
 
 // GetProviderName determines all AI service providers capable of serving a registered model.
@@ -59,6 +60,30 @@ func GetProviderName(modelName string) []string {
 	return providers
 }
 
+// ResolveAutoModel resolves the "auto" model name to an actual available model.
+// It uses an empty handler type to get any available model from the registry.
+//
+// Parameters:
+//   - modelName: The model name to check (should be "auto")
+//
+// Returns:
+//   - string: The resolved model name, or the original if not "auto" or resolution fails
+func ResolveAutoModel(modelName string) string {
+	if modelName != "auto" {
+		return modelName
+	}
+
+	// Use empty string as handler type to get any available model
+	firstModel, err := registry.GetGlobalRegistry().GetFirstAvailableModel("")
+	if err != nil {
+		log.Warnf("Failed to resolve 'auto' model: %v, falling back to original model name", err)
+		return modelName
+	}
+
+	log.Infof("Resolved 'auto' model to: %s", firstModel)
+	return firstModel
+}
+
 // IsOpenAICompatibilityAlias checks if the given model name is an alias
 // configured for OpenAI compatibility routing.
 //