✨ chore: Improve channel creation UX: defer "Fetch Model List" action until after creation

Previously, the "Fetch Model List" button was visible in the channel-creation view even though
it only functions once a channel record exists, leading to user confusion.

Changes introduced:
• Render the "Fetch Model List" button only when editing an existing channel (`isEdit === true`).
• Display an informational Banner in creation mode to remind users that the upstream model list
  can be fetched after the channel has been created.
• Refactored JSX to apply the above conditional rendering without altering existing logic.

This update streamlines the creation workflow and sets clearer expectations for users.

web/src/i18n/locales/en.json

@@ -1748,5 +1748,6 @@
   "快捷设置": "Quick settings",
   "批量创建时会在名称后自动添加随机后缀": "When creating in batches, a random suffix will be automatically added to the name",
   "额度必须大于0": "Quota must be greater than 0",
-  "生成数量必须大于0": "Generation quantity must be greater than 0"
+  "生成数量必须大于0": "Generation quantity must be greater than 0",
+  "创建后可在编辑渠道时获取上游模型列表": "After creation, you can get the upstream model list when editing the channel"
 }

web/src/pages/Channel/EditChannel.js

@@ -792,13 +792,15 @@ const EditChannel = (props) => {
                   >
                     {t('填入所有模型')}
                   </Button>
-                  <Button
-                    type='tertiary'
-                    onClick={() => fetchUpstreamModelList('models')}
-                    className="!rounded-lg"
-                  >
-                    {t('获取模型列表')}
-                  </Button>
+                  {isEdit ? (
+                    <Button
+                      type='tertiary'
+                      onClick={() => fetchUpstreamModelList('models')}
+                      className="!rounded-lg"
+                    >
+                      {t('获取模型列表')}
+                    </Button>
+                  ) : null}
                   <Button
                     type='warning'
                     onClick={() => handleInputChange('models', [])}
@@ -826,6 +828,14 @@ const EditChannel = (props) => {
                   </Button>
                 </div>
 
+                {!isEdit && (
+                  <Banner
+                    type='info'
+                    description={t('创建后可在编辑渠道时获取上游模型列表')}
+                    className='!rounded-lg'
+                  />
+                )}
+
                 <div>
                   <Input
                     addonAfter={