🐛 fix: preserve key data when editing channels & update MultiKeySize correctly

Backend
• `model/channel.go`
  – On multi-key channel updates, recalculate `MultiKeySize` from the current key list.
  – If the request omits `key`, fetch existing keys from DB to avoid resetting the count to `0/0`.
  – Remove out-of-range entries from `MultiKeyStatusList` to keep data consistent.

Frontend
• `web/src/pages/Channel/EditChannel.js`
  – Vertex AI channels no longer require re-uploading a key file in edit mode.
  – When no new key is provided during editing, exclude the `key` field from the payload to prevent overwriting the stored value.

These changes ensure that editing a channel no longer zeroes out the enabled key counter and that Vertex AI channels can be modified without mandatory key re-submission.

model/channel.go

@@ -398,7 +398,19 @@ func (channel *Channel) Insert() error {
 func (channel *Channel) Update() error {
 	// 如果是多密钥渠道，则根据当前 key 列表重新计算 MultiKeySize，避免编辑密钥后数量未同步
 	if channel.ChannelInfo.IsMultiKey {
-		keys := channel.getKeys()
+		var keyStr string
+		if channel.Key != "" {
+			keyStr = channel.Key
+		} else {
+			// 如果当前未提供 key，读取数据库中的现有 key
+			if existing, err := GetChannelById(channel.Id, true); err == nil {
+				keyStr = existing.Key
+			}
+		}
+		keys := []string{}
+		if keyStr != "" {
+			keys = strings.Split(strings.Trim(keyStr, "\n"), "\n")
+		}
 		channel.ChannelInfo.MultiKeySize = len(keys)
 		// 清理超过新 key 数量范围的状态数据，防止索引越界
 		if channel.ChannelInfo.MultiKeyStatusList != nil {