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_model_mapping.go

@@ -1,10 +1,8 @@
 package service
 
-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.
+// 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.
 func resolveOpenAIForwardModel(account *Account, requestedModel, defaultMappedModel string) string {
 	if account == nil {
 		if defaultMappedModel != "" {
@@ -19,23 +17,3 @@ 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"
-}