ci(wechat-notify): extract payload building logic to Python script

- Create new `.github/scripts/build_wechat_payload.py` to handle WeChat payload generation
- Replace inline Python string concatenation with dedicated script for better maintainability
- Add checkout step to access the script during workflow execution
- Simplify workflow by delegating payload construction to external script
- Improve code readability and reusability for future notification enhancements

.github/scripts/build_wechat_payload.py

@@ -0,0 +1,24 @@
+import json
+import os
+
+branch = os.environ.get("BRANCH", "")
+author = os.environ.get("AUTHOR", "")
+pr_title = os.environ.get("PR_TITLE", "")
+pr_url = os.environ.get("PR_URL", "")
+ai_summary = os.environ.get("AI_SUMMARY", "")
+
+content = (
+    "## 🚀 Release 发布通知\n"
+    f"> 📦 **分支**: {branch}\n"
+    f"> 👤 **提交人**: {author}\n"
+    f"> 📝 **标题**: {pr_title}\n\n"
+    "### 🧠 AI变更摘要\n"
+    f"{ai_summary}\n\n"
+    "---\n"
+    f"🔗 [查看PR详情]({pr_url})"
+)
+
+payload = {"msgtype": "markdown", "markdown": {"content": content}}
+
+with open("wechat_payload.json", "w", encoding="utf-8") as f:
+    json.dump(payload, f, ensure_ascii=False)