refactor: remove resolveOpenAIUpstreamModel, use normalizeCodexModel directly

Eliminates unnecessary indirection layer. The wrapper function only
called normalizeCodexModel with a special case for "gpt 5.3 codex spark"
(space-separated variant) that is no longer needed.

All call sites now use normalizeCodexModel directly.

backend/internal/service/openai_gateway_service.go

@@ -1939,7 +1939,7 @@ func (s *OpenAIGatewayService) Forward(ctx context.Context, c *gin.Context, acco
 
 	// 针对所有 OpenAI 账号执行 Codex 模型名规范化，确保上游识别一致。
 	if model, ok := reqBody["model"].(string); ok {
-		upstreamModel = resolveOpenAIUpstreamModel(model)
+		upstreamModel = normalizeCodexModel(model)
 		if upstreamModel != "" && upstreamModel != model {
 			logger.LegacyPrintf("service.openai_gateway", "[OpenAI] Upstream model resolved: %s -> %s (account: %s, type: %s, isCodexCLI: %v)",
 				model, upstreamModel, account.Name, account.Type, isCodexCLI)