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/handler/admin/channel_handler_test.go

@@ -273,13 +273,13 @@ func TestPricingRequestToService_Defaults(t *testing.T) {
 			wantValue: string(service.BillingModeToken),
 		},
 		{
-			name: "empty platform defaults to anthropic",
+			name: "empty platform stays empty",
 			req: channelModelPricingRequest{
 				Models:   []string{"m1"},
 				Platform: "",
 			},
 			wantField: "Platform",
-			wantValue: "anthropic",
+			wantValue: "",
 		},
 	}