fix(auth): rotate keys after DeepSeek insufficient balance

internal/clienterror/client_error.go

@@ -80,6 +80,12 @@ func IsRequestFault(status int, err error) bool {
 			status = statusErr.StatusCode()
 		}
 	}
+	// HTTP 402 is a credential payment or balance failure. Some upstreams,
+	// including DeepSeek, label it with the generic invalid_request_error code.
+	// The status is authoritative so the credential can cool down and rotate.
+	if status == http.StatusPaymentRequired {
+		return false
+	}
 	if hasRequestFaultBody(err) {
 		return true
 	}