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/payment/provider/easypay.go

@@ -276,12 +276,3 @@ func easyPaySign(params map[string]string, pkey string) string {
 func easyPayVerifySign(params map[string]string, pkey string, sign string) bool {
 	return hmac.Equal([]byte(easyPaySign(params, pkey)), []byte(sign))
 }
-
-// decodeURLValue URL-decodes a string once.
-func decodeURLValue(s string) string {
-	decoded, err := url.QueryUnescape(s)
-	if err != nil {
-		return s
-	}
-	return decoded
-}