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/pkg/websearch/manager_test.go

@@ -50,7 +50,7 @@ func TestManager_SearchWithBestProvider_UsesFirstAvailable(t *testing.T) {
 	srvBrave := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
 		resp := braveResponse{}
 		resp.Web.Results = []braveResult{{URL: "https://brave.com", Title: "Brave", Description: "from brave"}}
-		json.NewEncoder(w).Encode(resp)
+		_ = json.NewEncoder(w).Encode(resp)
 	}))
 	defer srvBrave.Close()
 
@@ -77,7 +77,7 @@ func TestManager_SearchWithBestProvider_NilRedis(t *testing.T) {
 	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
 		resp := braveResponse{}
 		resp.Web.Results = []braveResult{{URL: "https://test.com", Title: "Test", Description: "result"}}
-		json.NewEncoder(w).Encode(resp)
+		_ = json.NewEncoder(w).Encode(resp)
 	}))
 	defer srv.Close()