feat: support disabling image generation globally

- Added `disable-image-generation` configuration flag to disable the `image_generation` tool globally.
- Updated payload handling to remove `image_generation` tools from request payload arrays when the flag is enabled.
- Modified OpenAI image handlers (`ImagesGenerations`, `ImagesEdits`) to return 404 when the feature is disabled.
- Enhanced configuration diff logging to track changes for the `disable-image-generation` flag.
- Added accompanying unit tests for the new feature in payload helpers and image handler logic.

internal/api/server.go

@@ -1013,6 +1013,10 @@ func (s *Server) UpdateClients(cfg *config.Config) {
 		auth.SetQuotaCooldownDisabled(cfg.DisableCooling)
 	}
 
+	if oldCfg != nil && oldCfg.DisableImageGeneration != cfg.DisableImageGeneration {
+		log.Infof("disable-image-generation updated: %t -> %t", oldCfg.DisableImageGeneration, cfg.DisableImageGeneration)
+	}
+
 	applySignatureCacheConfig(oldCfg, cfg)
 
 	if s.handlers != nil && s.handlers.AuthManager != nil {