Use PromptKit prepared execution for structured model calls

internal/framework/llm/secrets.go

@@ -8,10 +8,11 @@ func ConfiguredSecrets(cfg *config.Config) []string {
 	if cfg == nil {
 		return nil
 	}
-	effectiveValidation := cfg.EffectiveValidationLLMConfig()
-	return []string{
-		cfg.PrimaryLLM.APIKey,
-		cfg.ValidationLLM.APIKey,
-		effectiveValidation.APIKey,
+	secrets := make([]string, 0, len(cfg.Backends))
+	for _, backend := range cfg.Backends {
+		if backend.Credential != nil && backend.Credential.Value() != "" {
+			secrets = append(secrets, backend.Credential.Value())
+		}
 	}
+	return secrets
 }