fix(钉钉通知): 修复 Markdown 消息换行被折叠导致排版错乱的问题

Motivation:
钉钉 Markdown 渲染时会将单个换行符折叠为空格，导致推送的通知内容被压缩成一行，严重影响消息可读性。

Changes:

* 发送前将消息正文中的单个换行统一转换为段落分隔（\n\n），确保模板换行在钉钉中正确渲染
* 归一化 \r\n 与 \r 换行，并合并已有连续换行，避免产生多余空行
* 新增换行转换逻辑的单元测试

internal/adapter/dingtalk.go

@@ -10,6 +10,7 @@ import (
 	"fmt"
 	"net/http"
 	"net/url"
+	"strings"
 	"time"
 )
 
@@ -69,7 +70,7 @@ func (s *DingTalkSender) Send(title, content string, config json.RawMessage) err
 		MsgType: "markdown",
 		Markdown: &dingtalkMD{
 			Title: title,
-			Text:  content,
+			Text:  dingtalkMarkdownText(content),
 		},
 	}
 
@@ -86,6 +87,18 @@ func (s *DingTalkSender) Send(title, content string, config json.RawMessage) err
 	return nil
 }
 
+// dingtalkMarkdownText turns template newlines into DingTalk markdown paragraph
+// breaks. A single \n is collapsed to a space by DingTalk markdown, so each
+// logical line must be separated by \n\n.
+func dingtalkMarkdownText(content string) string {
+	content = strings.ReplaceAll(content, "\r\n", "\n")
+	content = strings.ReplaceAll(content, "\r", "\n")
+	for strings.Contains(content, "\n\n") {
+		content = strings.ReplaceAll(content, "\n\n", "\n")
+	}
+	return strings.ReplaceAll(content, "\n", "\n\n")
+}
+
 func dingtalkSign(timestamp int64, secret string) string {
 	mac := hmac.New(sha256.New, []byte(secret))
 	fmt.Fprintf(mac, "%d\n%s", timestamp, secret)