[BUG] method renderField in message_tpl.go will cause the alert info to generate line break error on third-party platform

### Question and Steps to reproduce

These two lines of code escape all newline characters in the alert event, and then send the alert message to third-party platforms. After the third-party receives and sends the message, the newlines are displayed directly as \n instead of actual line breaks.
`escaped = strings.ReplaceAll(escaped, "\n", "\\n")
escaped = strings.ReplaceAll(escaped, "\r", "\\r") `

`func (t *MessageTemplate) renderField(key, msgTpl string, renderData map[string]interface{}) (interface{}, error) {
text := strings.Join(append(GetDefs(renderData), msgTpl), "")

if t.NotifyChannelIdent == "email" {
tpl, err := texttemplate.New(key).Funcs(tplx.TemplateFuncMap).Parse(text)
if err != nil {
return nil, fmt.Errorf("failed to parse template: %v", err)
}
var body bytes.Buffer
if err = tpl.Execute(&body, renderData); err != nil {
return nil, fmt.Errorf("failed to execute template: %v", err)
}
return body.String(), nil
}

tpl, err := template.New(key).Funcs(tplx.TemplateFuncMap).Parse(text)
if err != nil {
return nil, fmt.Errorf("failed to parse template: %v", err)
}
var body bytes.Buffer
if err = tpl.Execute(&body, renderData); err != nil {
return nil, fmt.Errorf("failed to execute template: %v", err)
}

escaped := strings.ReplaceAll(body.String(), `"`, `\"`)
escaped = strings.ReplaceAll(escaped, "\n", "\\n")
escaped = strings.ReplaceAll(escaped, "\r", "\\r")
if isSlackIdent(t.NotifyChannelIdent) {
escaped = strings.ReplaceAll(escaped, "<", "<")
}
return template.HTML(escaped), nil
}`

### Relevant logs and configurations

```text
平台接收到的告警： 级别状态: S3 Triggered \n规则标题: 服务器通用告警-主机内存不足 \n规则备注: 主机 xx 内存使用率为 85.75% \n监控指标: [cluster=xx rulename=服务器通用告警-主机内存不足] \n触发时值: 85.75 \n首次触发时间: 2026-09-01 19:25:09 \n距离首次告警: 21h 13m 4s\n发送时间: 2026-09-02 16:38:13
```

### Version

v9.1.1

Start in message_tpl.go at MessageTemplate.renderField and inspect the newline escaping applied after template execution for non-email channels. Reproduce the alert with multiline content on a third-party platform, then verify that received messages show actual line breaks while the existing email and Slack handling remains correct.

Written by the indexing model from the issue text.