fix(service): 消除 isOpenAIGPT56Model 重复声明，统一到 openai_model_alias.go

PR #3898 与 #3909 各自新增了同名函数（billing 精确匹配版 / gateway
前缀匹配版），合并后 internal/service 编译失败。保留前缀匹配语义作为
唯一实现：对 normalizeKnownOpenAICodexModel 的输出（gpt-5.6-* 精确
基名）canonicalize 幂等且精确分支命中，计费行为不变；同时保住
gpt-5.6-*-max 等后缀变体的识别。

backend/internal/service/billing_service.go

@@ -1141,10 +1141,6 @@ func (s *BillingService) shouldApplySessionLongContextPricing(tokens UsageTokens
 	return totalInputTokens > pricing.LongContextInputThreshold
 }
 
-func isOpenAIGPT56Model(normalized string) bool {
-	return normalized == "gpt-5.6-sol" || normalized == "gpt-5.6-terra" || normalized == "gpt-5.6-luna"
-}
-
 func usesOpenAILegacyLongContextPricing(normalized string) bool {
 	return normalized == "gpt-5.4" || normalized == "gpt-5.5" || normalized == "gpt-5.5-pro"
 }

backend/internal/service/openai_gateway_request_body.go

@@ -1218,13 +1218,3 @@ func normalizeOpenAIReasoningEffortForModel(raw, model string) string {
 	}
 	return normalizeOpenAIReasoningEffort(raw)
 }
-
-func isOpenAIGPT56Model(model string) bool {
-	normalized := canonicalizeOpenAIModelAliasSpelling(model)
-	for _, prefix := range []string{"gpt-5.6-sol", "gpt-5.6-terra", "gpt-5.6-luna"} {
-		if normalized == prefix || strings.HasPrefix(normalized, prefix+"-") {
-			return true
-		}
-	}
-	return false
-}

backend/internal/service/openai_model_alias.go

@@ -98,6 +98,18 @@ func normalizeKnownOpenAICodexModel(model string) string {
 	}
 }
 
+// isOpenAIGPT56Model 判断是否 GPT-5.6 系列模型；入参可为原始模型名
+// （含大小写/路径/后缀变体）或已归一化的基名，两者均能正确识别。
+func isOpenAIGPT56Model(model string) bool {
+	normalized := canonicalizeOpenAIModelAliasSpelling(model)
+	for _, prefix := range []string{"gpt-5.6-sol", "gpt-5.6-terra", "gpt-5.6-luna"} {
+		if normalized == prefix || strings.HasPrefix(normalized, prefix+"-") {
+			return true
+		}
+	}
+	return false
+}
+
 func appendUsageBillingModelCandidate(candidates []string, seen map[string]struct{}, model string) []string {
 	trimmed := strings.TrimSpace(model)
 	if trimmed == "" {