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/gemini_v1beta_handler.go

@@ -682,6 +682,16 @@ func shouldFallbackGeminiModels(res *service.UpstreamHTTPResult) bool {
 	return false
 }
 
+func shouldFallbackGeminiModel(modelName string, res *service.UpstreamHTTPResult) bool {
+	if shouldFallbackGeminiModels(res) {
+		return true
+	}
+	if res == nil || res.StatusCode != http.StatusNotFound {
+		return false
+	}
+	return gemini.HasFallbackModel(modelName)
+}
+
 // extractGeminiCLISessionHash 从 Gemini CLI 请求中提取会话标识。
 // 组合 x-gemini-api-privileged-user-id header 和请求体中的 tmp 目录哈希。
 //