fix: resolve upstream CI failures (lint, test, gofmt)

- Fix errcheck: handle Write/Encode return values in brave_test.go
- Fix errcheck: defer resp.Body.Close() with _ assignment in tavily.go
- Fix gofmt: payment.go, channel.go, payment_config_providers.go
- Fix unused: remove dead decodeURLValue in easypay.go
- Restore shouldFallbackGeminiModel function (deleted during cherry-pick)
- Add missing balanceNotifyService param to NewGatewayService in test
- Fix platform default test expectation (empty stays empty)
- Fix wildcard pricing test (longest prefix wins, not config order)
- Fix subscription group test (SUBSCRIPTION_REPOSITORY_UNAVAILABLE)

backend/internal/service/payment_config_providers.go

@@ -22,16 +22,16 @@ func (s *PaymentConfigService) ListProviderInstances(ctx context.Context) ([]*db
 
 // ProviderInstanceResponse is the API response for a provider instance.
 type ProviderInstanceResponse struct {
-	ID              int64             `json:"id"`
-	ProviderKey     string            `json:"provider_key"`
-	Name            string            `json:"name"`
-	Config          map[string]string `json:"config"`
-	SupportedTypes  []string          `json:"supported_types"`
-	Limits          string            `json:"limits"`
-	Enabled         bool              `json:"enabled"`
-	RefundEnabled   bool              `json:"refund_enabled"`
-	SortOrder       int               `json:"sort_order"`
-	PaymentMode     string            `json:"payment_mode"`
+	ID             int64             `json:"id"`
+	ProviderKey    string            `json:"provider_key"`
+	Name           string            `json:"name"`
+	Config         map[string]string `json:"config"`
+	SupportedTypes []string          `json:"supported_types"`
+	Limits         string            `json:"limits"`
+	Enabled        bool              `json:"enabled"`
+	RefundEnabled  bool              `json:"refund_enabled"`
+	SortOrder      int               `json:"sort_order"`
+	PaymentMode    string            `json:"payment_mode"`
 }
 
 // ListProviderInstancesWithConfig returns provider instances with decrypted config.