✨ feat: enable fetching model list in creation mode & refine toast-based error handling

Summary
• **EditChannel.js**
  – Displays “Fetch Model List” button for both *create* and *edit* modes (removed `isEdit` guard).
  – Unified model selector placeholder to “Please choose supported models”.
  – Added null-safety checks when parsing Axios responses.
  – Sends requests with `{ skipErrorHandler: true }`, preventing generic *500 Internal Server Error* toasts and relying on context-specific messages instead.

• **helpers/api.js**
  – Introduced `skipErrorHandler` flag in the Axios response interceptor.
    If present, global `showError` is bypassed, giving callers full control over user-facing notifications.
  – Ensures `Promise.reject(error)` is returned for proper error propagation.

Why
Channel creators now enjoy the same convenience as editors when importing upstream model lists.
Meanwhile, suppressing redundant toasts removes confusion caused by simultaneous custom and generic error messages.

web/src/pages/Channel/EditChannel.js

@@ -238,9 +238,9 @@ const EditChannel = (props) => {
     let err = false;
 
     if (isEdit) {
-      // 如果是编辑模式，使用已有的channel id获取模型列表
-      const res = await API.get('/api/channel/fetch_models/' + channelId);
-      if (res.data && res.data.success) {
+      // 如果是编辑模式，使用已有的 channelId 获取模型列表
+      const res = await API.get('/api/channel/fetch_models/' + channelId, { skipErrorHandler: true });
+      if (res && res.data && res.data.success) {
         models.push(...res.data.data);
       } else {
         err = true;
@@ -252,13 +252,17 @@ const EditChannel = (props) => {
         err = true;
       } else {
         try {
-          const res = await API.post('/api/channel/fetch_models', {
-            base_url: inputs['base_url'],
-            type: inputs['type'],
-            key: inputs['key'],
-          });
+          const res = await API.post(
+            '/api/channel/fetch_models',
+            {
+              base_url: inputs['base_url'],
+              type: inputs['type'],
+              key: inputs['key'],
+            },
+            { skipErrorHandler: true },
+          );
 
-          if (res.data && res.data.success) {
+          if (res && res.data && res.data.success) {
             models.push(...res.data.data);
           } else {
             err = true;
@@ -747,7 +751,7 @@ const EditChannel = (props) => {
                   <Form.Select
                     field='models'
                     label={t('模型')}
-                    placeholder={isEdit ? t('请选择该渠道所支持的模型') : t('创建后可在编辑渠道时获取上游模型列表')}
+                    placeholder={t('请选择该渠道所支持的模型')}
                     rules={[{ required: true, message: t('请选择模型') }]}
                     multiple
                     filter
@@ -763,11 +767,9 @@ const EditChannel = (props) => {
                         <Button size='small' type='secondary' onClick={() => handleInputChange('models', fullModels)}>
                           {t('填入所有模型')}
                         </Button>
-                        {isEdit && (
-                          <Button size='small' type='tertiary' onClick={() => fetchUpstreamModelList('models')}>
-                            {t('获取模型列表')}
-                          </Button>
-                        )}
+                        <Button size='small' type='tertiary' onClick={() => fetchUpstreamModelList('models')}>
+                          {t('获取模型列表')}
+                        </Button>
                         <Button size='small' type='warning' onClick={() => handleInputChange('models', [])}>
                           {t('清除所有模型')}
                         </Button>