修复md2img运行时并发单例问题，避免预热双启动日志

utils/markdown_to_image.py

@@ -689,6 +689,7 @@ class _PersistentBrowser:
 
 _BROWSER_MANAGER: Optional[_PersistentBrowser] = None
 _MD2IMG_RUNTIME = None
+_MD2IMG_RUNTIME_LOCK = threading.Lock()
 
 
 class _Md2ImgRuntime:
@@ -754,8 +755,12 @@ class _Md2ImgRuntime:
 
 def _get_md2img_runtime() -> _Md2ImgRuntime:
     global _MD2IMG_RUNTIME
+    # 并发首次访问时要加锁，避免创建出多个 runtime 实例，
+    # 进而出现“专用运行时已启动”日志重复与多线程并存问题。
     if _MD2IMG_RUNTIME is None:
-        _MD2IMG_RUNTIME = _Md2ImgRuntime()
+        with _MD2IMG_RUNTIME_LOCK:
+            if _MD2IMG_RUNTIME is None:
+                _MD2IMG_RUNTIME = _Md2ImgRuntime()
     return _MD2IMG_RUNTIME