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/pkg/apicompat/chatcompletions_to_responses.go

@@ -33,7 +33,7 @@ func ChatCompletionsToResponses(req *ChatCompletionsRequest) (*ResponsesRequest,
 		TopP:        req.TopP,
 		Stream:      true, // upstream always streams
 		Include:     []string{"reasoning.encrypted_content"},
-		ServiceTier:  req.ServiceTier,
+		ServiceTier: req.ServiceTier,
 	}
 
 	storeFalse := false