refactor: extract block dnd handle predicate

rust/spikes/leptos-tiptap-spike/src/editor_runtime/block_dnd.rs

@@ -4,7 +4,7 @@
 //! 也不分发 editor command；调用方负责保留编辑器状态编排。
 
 use crate::editor_runtime::block_hover_state::{DropIndicatorState, DropPlacement};
-use web_sys::{window, EventTarget};
+use web_sys::{window, Element, EventTarget};
 
 pub(crate) struct HandleCorridorGeometry {
     pub(crate) stage_left: f64,
@@ -81,6 +81,10 @@ pub(crate) fn event_target_from_point(client_x: i32, client_y: i32) -> Option<Ev
         .map(Into::into)
 }
 
+pub(crate) fn element_within_handle_shell(element: &Element, handle_selector: &str) -> bool {
+    element.closest(handle_selector).ok().flatten().is_some()
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;