Fix the bug in enablePrologue under agent task mode (#9487)

### What problem does this PR solve?

There is a problem with the implementation of the Agent begin-form:
although the enablePrologue switch and the prologue input box are hidden
in Task mode, these values are still saved in the form data. If the user
first enables the opening and sets the content in Conversational mode,
and then switches to Task mode, these values will still be saved and may
be used in some scenarios.
### Type of change

- [x] Bug Fix (non-breaking change which fixes an issue)

web/src/pages/agent/form/begin-form/index.tsx

 import { buildSelectOptions } from '@/utils/component-util';
 import { zodResolver } from '@hookform/resolvers/zod';
 import { Plus } from 'lucide-react';
-import { memo } from 'react';
+import { memo, useEffect, useRef } from 'react';
 import { useForm, useWatch } from 'react-hook-form';
 import { useTranslation } from 'react-i18next';
 import { z } from 'zod';
     name: 'enablePrologue',
   });
 
+  const previousModeRef = useRef(mode);
+
+  useEffect(() => {
+    if (
+      previousModeRef.current === AgentDialogueMode.Task &&
+      mode === AgentDialogueMode.Conversational
+    ) {
+      form.setValue('enablePrologue', true);
+    }
+    previousModeRef.current = mode;
+  }, [mode, form]);
+
   const {
     ok,
     currentRecord,