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/channel.go

@@ -37,8 +37,8 @@ type Channel struct {
 	Name               string
 	Description        string
 	Status             string
-	BillingModelSource string // "requested", "upstream", or "channel_mapped"
-	RestrictModels     bool   // 是否限制模型（仅允许定价列表中的模型）
+	BillingModelSource string         // "requested", "upstream", or "channel_mapped"
+	RestrictModels     bool           // 是否限制模型（仅允许定价列表中的模型）
 	Features           string         // 渠道特性描述（JSON 数组），用于支付页面展示
 	FeaturesConfig     map[string]any // 渠道功能配置（如 web search emulation）
 	CreatedAt          time.Time