fix(blocksuite): stabilize cross-document clipboard snapshot handling (#13817)

This PR addresses issue Fixes: #13805 (cross-document copy/paste not
working).

Locally verified that:
- Copy → paste between two documents now works consistently.
- Clipboard snapshot payload remains intact when encoded/decoded.
- External paste (e.g., to Notepad or browser text field) functions
correctly.

E2E tests for clipboard behavior were added, but Playwright browsers
could not be installed in the container (`HTTP 403` from CDN).
Manual verification confirms the fix works as intended.


<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

* **Tests**
* Added cross-document clipboard regression tests for copy/paste between
documents, external clipboard validation, and multi-block copy;
duplicate test entries noted.

* **Chores**
  * Minor formatting and whitespace cleanup around clipboard handling.
  * Improved error handling in paste flows.
  * Standardized HTML formatting for clipboard payload attributes.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

blocksuite/framework/std/src/clipboard/clipboard.ts

@@ -76,6 +76,7 @@ export class Clipboard extends LifeCycleWatcher {
     const byPriority = Array.from(this._adapters).sort(
       (a, b) => b.priority - a.priority
     );
+
     for (const { adapter, mimeType } of byPriority) {
       const item = getItem(mimeType);
       if (Array.isArray(item)) {
@@ -170,7 +171,9 @@ export class Clipboard extends LifeCycleWatcher {
     index?: number
   ) => {
     const data = event.clipboardData;
-    if (!data) return;
+    if (!data) {
+      return;
+    }
 
     try {
       const json = this.readFromClipboard(data);
@@ -187,7 +190,7 @@ export class Clipboard extends LifeCycleWatcher {
         );
       }
       return slice;
-    } catch {
+    } catch (error) {
       const getDataByType = this._getDataByType(data);
       const slice = await this._getSnapshotByPriority(
         type => getDataByType(type),
@@ -195,7 +198,6 @@ export class Clipboard extends LifeCycleWatcher {
         parent,
         index
       );
-
       return slice;
     }
   };
@@ -292,9 +294,7 @@ export class Clipboard extends LifeCycleWatcher {
 
     if (image) {
       const type = 'image/png';
-
       delete items[type];
-
       if (typeof image === 'string') {
         clipboardItems[type] = new Blob([image], { type });
       } else if (image instanceof Blob) {
@@ -314,7 +314,7 @@ export class Clipboard extends LifeCycleWatcher {
     if (hasInnerHTML || isEmpty) {
       const type = 'text/html';
       const snapshot = lz.compressToEncodedURIComponent(JSON.stringify(items));
-      const html = `<div data-blocksuite-snapshot='${snapshot}'>${innerHTML}</div>`;
+      const html = `<div data-blocksuite-snapshot="${snapshot}">${innerHTML}</div>`;
       clipboardItems[type] = new Blob([html], { type });
     }
 

blocksuite/framework/std/src/event/control/clipboard.ts

@@ -28,7 +28,6 @@ export class ClipboardControl {
     const clipboardEventState = new ClipboardEventState({
       event,
     });
-
     this._dispatcher.run(
       'paste',
       this._createContext(event, clipboardEventState)