🚑 fix: safeguard NewAPIError.Error() against nil pointer panic

Backend
• `types/error.go`
  – Return empty string when receiver itself is `nil`.
  – If `Err` is `nil`, fall back to `errorCode` string to avoid calling `nil.Error()`.

This prevents runtime panics when the error handler builds an OpenAI-style error response but the underlying `Err` field has not been set.

types/error.go

@@ -88,6 +88,13 @@ func (e *NewAPIError) GetErrorCode() ErrorCode {
 }
 
 func (e *NewAPIError) Error() string {
+	if e == nil {
+		return ""
+	}
+	if e.Err == nil {
+		// fallback message when underlying error is missing
+		return string(e.errorCode)
+	}
 	return e.Err.Error()
 }