Feat: Fixed the issue where the newly created agent begin node displayed "undefined". #10427 (#12348)

### What problem does this PR solve?
Feat: Fixed the issue where the newly created agent begin node displayed
"undefined". #10427

### Type of change


- [x] New Feature (non-breaking change which adds functionality)

web/src/constants/agent.tsx

@@ -211,3 +211,14 @@ export const WebhookJWTAlgorithmList = [
   'ps512',
   'none',
 ] as const;
+
+export enum AgentDialogueMode {
+  Conversational = 'conversational',
+  Task = 'task',
+  Webhook = 'Webhook',
+}
+
+export const initialBeginValues = {
+  mode: AgentDialogueMode.Conversational,
+  prologue: `Hi! I'm your assistant. What can I do for you?`,
+};