Encode generation payloads once on success

internal/llm/openai_compatible_client.go

@@ -152,6 +152,11 @@ func (c *OpenAICompatibleClient) PrepareGeneration(req domain.GenerateRequest) (
 
 	payload, err := json.Marshal(wirePayload)
 	if err != nil {
+		for key, value := range req.Target.ExtraParams {
+			if _, extraErr := json.Marshal(value); extraErr != nil {
+				return nil, fmt.Errorf("%w: extra_params.%s must be JSON-serializable: %v", ErrInvalidRequest, key, extraErr)
+			}
+		}
 		return nil, fmt.Errorf("%w: failed to encode request: %v", ErrRequestFailed, err)
 	}
 	apiKey := strings.TrimSpace(req.Target.APIKey)
@@ -446,9 +451,6 @@ func openAIChatRequestPayload(req openAIChatRequest) (map[string]any, error) {
 		if IsReservedOpenAIChatRequestField(key) {
 			return nil, fmt.Errorf("extra_params key %q collides with reserved request field", key)
 		}
-		if _, err := json.Marshal(value); err != nil {
-			return nil, fmt.Errorf("extra_params.%s must be JSON-serializable: %w", key, err)
-		}
 		out[key] = value
 	}