fix: resolve remaining lint errors for upstream CI

- Fix errcheck: brave.go resp.Body.Close, manager_test.go Encode
- Fix gofmt: payment_config_service.go
- Fix unused: use shouldFallbackGeminiModel (with modelName param) in handler

backend/internal/handler/gemini_v1beta_handler.go

@@ -121,7 +121,7 @@ func (h *GatewayHandler) GeminiV1BetaGetModel(c *gin.Context) {
 		googleError(c, http.StatusBadGateway, err.Error())
 		return
 	}
-	if shouldFallbackGeminiModels(res) {
+	if shouldFallbackGeminiModel(modelName, res) {
 		c.JSON(http.StatusOK, gemini.FallbackModel(modelName))
 		return
 	}