fix: 修复企业微信通知环境变量展开问题

- 使用 PAYLOAD=$(cat <<ENDPAYLOAD) 替代 cat > file <<EOF
- 确保环境变量在 heredoc 中正确展开
- 添加测试脚本验证环境变量展开
- 修复构建详情链接和消息内容缺失问题

.woodpecker.yml

@@ -424,14 +424,17 @@ steps:
         REPO_ID="${CI_REPO_ID:-1}"
         TIMESTAMP=$(date "+%Y-%m-%d %H:%M:%S")
         
-        cat > /tmp/payload.json <<EOF
+        PAYLOAD=$(cat <<ENDPAYLOAD
         {
           "msgtype": "markdown",
           "markdown": {
             "content": "## 🚀 Novalon Website 部署通知\n\n> **构建状态**: <font color=\"info\">成功</font>\n\n**项目信息**\n> 分支: \`${BRANCH}\`\n> 提交: \`${COMMIT}\`\n> 作者: ${AUTHOR}\n\n**提交信息**\n> ${MESSAGE}\n\n**操作**\n> [查看构建详情](https://ci.f.novalon.cn/repos/${REPO_ID}/pipeline/${PIPELINE_NUMBER})\n\n---\n> 时间: ${TIMESTAMP}\n> Pipeline #${PIPELINE_NUMBER}"
           }
         }
-        EOF
+        ENDPAYLOAD
+        )
+        
+        echo "$PAYLOAD" > /tmp/payload.json
         
         curl -X POST "$WECHAT_WEBHOOK" \
           -H 'Content-Type: application/json' \
@@ -460,14 +463,17 @@ steps:
         REPO_ID="${CI_REPO_ID:-1}"
         TIMESTAMP=$(date "+%Y-%m-%d %H:%M:%S")
         
-        cat > /tmp/payload.json <<EOF
+        PAYLOAD=$(cat <<ENDPAYLOAD
         {
           "msgtype": "markdown",
           "markdown": {
             "content": "## 🚀 Novalon Website 部署通知\n\n> **构建状态**: <font color=\"warning\">失败</font>\n\n**项目信息**\n> 分支: \`${BRANCH}\`\n> 提交: \`${COMMIT}\`\n> 作者: ${AUTHOR}\n\n**提交信息**\n> ${MESSAGE}\n\n**操作**\n> [查看构建详情](https://ci.f.novalon.cn/repos/${REPO_ID}/pipeline/${PIPELINE_NUMBER})\n\n---\n> 时间: ${TIMESTAMP}\n> Pipeline #${PIPELINE_NUMBER}"
           }
         }
-        EOF
+        ENDPAYLOAD
+        )
+        
+        echo "$PAYLOAD" > /tmp/payload.json
         
         curl -X POST "$WECHAT_WEBHOOK" \
           -H 'Content-Type: application/json' \