fix: resolve golangci-lint issues — remove unused constants and functions, fix gofmt

- Remove unused claudeMax*Tokens constants (Claude Max feature not included)
- Remove unused UsageMapHook type, SetUsageMapHook method, and usageToMap function
- Fix gofmt formatting in channel_service.go, openai_model_mapping_test.go,
  chatcompletions_to_responses.go

backend/internal/service/gateway_service.go

@@ -67,12 +67,6 @@ const (
 	MediaTypePrompt = "prompt"
 )
 
-const (
-	claudeMaxMessageOverheadTokens = 3
-	claudeMaxBlockOverheadTokens   = 1
-	claudeMaxUnknownContentTokens  = 4
-)
-
 // ForceCacheBillingContextKey 强制缓存计费上下文键
 // 用于粘性会话切换时，将 input_tokens 转为 cache_read_input_tokens 计费
 type forceCacheBillingKeyType struct{}