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/antigravity/stream_transformer.go

@@ -18,9 +18,6 @@ const (
 	BlockTypeFunction
 )
 
-// UsageMapHook is a callback that can modify usage data before it's emitted in SSE events.
-type UsageMapHook func(usageMap map[string]any)
-
 // StreamingProcessor 流式响应处理器
 type StreamingProcessor struct {
 	blockType         BlockType
@@ -33,7 +30,6 @@ type StreamingProcessor struct {
 	originalModel     string
 	webSearchQueries  []string
 	groundingChunks   []GeminiGroundingChunk
-	usageMapHook      UsageMapHook
 
 	// 累计 usage
 	inputTokens       int
@@ -50,28 +46,6 @@ func NewStreamingProcessor(originalModel string) *StreamingProcessor {
 	}
 }
 
-// SetUsageMapHook sets an optional hook that modifies usage maps before they are emitted.
-func (p *StreamingProcessor) SetUsageMapHook(fn UsageMapHook) {
-	p.usageMapHook = fn
-}
-
-func usageToMap(u ClaudeUsage) map[string]any {
-	m := map[string]any{
-		"input_tokens":  u.InputTokens,
-		"output_tokens": u.OutputTokens,
-	}
-	if u.CacheCreationInputTokens > 0 {
-		m["cache_creation_input_tokens"] = u.CacheCreationInputTokens
-	}
-	if u.CacheReadInputTokens > 0 {
-		m["cache_read_input_tokens"] = u.CacheReadInputTokens
-	}
-	if u.ImageOutputTokens > 0 {
-		m["image_output_tokens"] = u.ImageOutputTokens
-	}
-	return m
-}
-
 // ProcessLine 处理 SSE 行，返回 Claude SSE 事件
 func (p *StreamingProcessor) ProcessLine(line string) []byte {
 	line = strings.TrimSpace(line)