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/watcher/diff/config_diff.go

@@ -42,6 +42,9 @@ func BuildConfigChangeDetails(oldCfg, newCfg *config.Config) []string {
 	if oldCfg.DisableCooling != newCfg.DisableCooling {
 		changes = append(changes, fmt.Sprintf("disable-cooling: %t -> %t", oldCfg.DisableCooling, newCfg.DisableCooling))
 	}
+	if oldCfg.DisableImageGeneration != newCfg.DisableImageGeneration {
+		changes = append(changes, fmt.Sprintf("disable-image-generation: %t -> %t", oldCfg.DisableImageGeneration, newCfg.DisableImageGeneration))
+	}
 	if oldCfg.RequestLog != newCfg.RequestLog {
 		changes = append(changes, fmt.Sprintf("request-log: %t -> %t", oldCfg.RequestLog, newCfg.RequestLog))
 	}