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/channel_service.go

@@ -134,7 +134,7 @@ func (r ChannelMappingResult) ToUsageFields(reqModel, upstreamModel string) Chan
 
 const (
 	channelCacheTTL       = 10 * time.Minute
-	channelErrorTTL       = 5 * time.Second  // DB 错误时的短缓存
+	channelErrorTTL       = 5 * time.Second // DB 错误时的短缓存
 	channelCacheDBTimeout = 10 * time.Second
 )
 

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{}

backend/internal/service/openai_model_mapping_test.go

@@ -87,10 +87,10 @@ func TestResolveOpenAIForwardModel_PreventsClaudeModelFromFallingBackToGpt51(t *
 
 func TestNormalizeCodexModel(t *testing.T) {
 	cases := map[string]string{
-		"gpt-5.3-codex-spark":          "gpt-5.3-codex",
-		"gpt-5.3-codex-spark-high":     "gpt-5.3-codex",
-		"gpt-5.3-codex-spark-xhigh":    "gpt-5.3-codex",
-		"gpt-5.3":                      "gpt-5.3-codex",
+		"gpt-5.3-codex-spark":       "gpt-5.3-codex",
+		"gpt-5.3-codex-spark-high":  "gpt-5.3-codex",
+		"gpt-5.3-codex-spark-xhigh": "gpt-5.3-codex",
+		"gpt-5.3":                   "gpt-5.3-codex",
 	}
 
 	for input, expected := range cases {