refactor: improve model resolution and normalization logic for OpenAI integration

backend/internal/service/openai_model_mapping.go

@@ -1,8 +1,10 @@
 package service
 
-// resolveOpenAIForwardModel determines the upstream model for OpenAI-compatible
-// forwarding. Group-level default mapping only applies when the account itself
-// did not match any explicit model_mapping rule.
+import "strings"
+
+// resolveOpenAIForwardModel resolves the account/group mapping result for
+// OpenAI-compatible forwarding. Group-level default mapping only applies when
+// the account itself did not match any explicit model_mapping rule.
 func resolveOpenAIForwardModel(account *Account, requestedModel, defaultMappedModel string) string {
 	if account == nil {
 		if defaultMappedModel != "" {
@@ -17,3 +19,23 @@ func resolveOpenAIForwardModel(account *Account, requestedModel, defaultMappedMo
 	}
 	return mappedModel
 }
+
+func resolveOpenAIUpstreamModel(model string) string {
+	if isBareGPT53CodexSparkModel(model) {
+		return "gpt-5.3-codex-spark"
+	}
+	return normalizeCodexModel(strings.TrimSpace(model))
+}
+
+func isBareGPT53CodexSparkModel(model string) bool {
+	modelID := strings.TrimSpace(model)
+	if modelID == "" {
+		return false
+	}
+	if strings.Contains(modelID, "/") {
+		parts := strings.Split(modelID, "/")
+		modelID = parts[len(parts)-1]
+	}
+	normalized := strings.ToLower(strings.TrimSpace(modelID))
+	return normalized == "gpt-5.3-codex-spark" || normalized == "gpt 5.3 codex spark"
+}