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 -->

tests/affine-local/e2e/blocksuite/clipboard/clipboard.spec.ts

@@ -474,6 +474,79 @@ test.describe('paste in readonly mode', () => {
   });
 });
 
+test.describe('cross document clipboard regression', () => {
+  test('copy and paste paragraph content between docs', async ({ page }) => {
+    const container = locateEditorContainer(page);
+    await container.click();
+
+    const sourceText = "Cross-doc paste can't fail again";
+    await type(page, sourceText);
+
+    const { blockIds } = await getParagraphIds(page);
+    await setSelection(page, blockIds[0], 0, blockIds[0], sourceText.length);
+
+    await copyByKeyboard(page);
+
+    await clickNewPageButton(page, 'Clipboard Destination');
+    await waitForEditorLoad(page);
+
+    const destination = locateEditorContainer(page);
+    await destination.click();
+
+    await pasteByKeyboard(page);
+    await page.waitForTimeout(100);
+
+    const pastedTexts = await page.locator(paragraphLocator).allTextContents();
+    expect(pastedTexts.some(text => text.includes(sourceText))).toBe(true);
+  });
+
+  test('copied content remains available to external clipboard consumers', async ({
+    page,
+  }) => {
+    const container = locateEditorContainer(page);
+    await container.click();
+
+    const textForExternal = 'External clipboard visibility check';
+    await type(page, textForExternal);
+
+    const { blockIds } = await getParagraphIds(page);
+    await setSelection(
+      page,
+      blockIds[0],
+      0,
+      blockIds[0],
+      textForExternal.length
+    );
+
+    await copyByKeyboard(page);
+
+    const plainText = await page.evaluate(() => navigator.clipboard.readText());
+
+    expect(plainText).toBe(textForExternal);
+  });
+
+  test('copy and paste within a single document still duplicates content', async ({
+    page,
+  }) => {
+    const container = locateEditorContainer(page);
+    await container.click();
+
+    const intraDocText = 'Same doc paste regression guard';
+    await type(page, intraDocText);
+
+    const { blockIds } = await getParagraphIds(page);
+    await setSelection(page, blockIds[0], 0, blockIds[0], intraDocText.length);
+
+    await copyByKeyboard(page);
+
+    await pressEnter(page);
+    await pasteByKeyboard(page);
+    await page.waitForTimeout(100);
+
+    await verifyParagraphContent(page, 1, intraDocText);
+  });
+});
+
 test('should copy single image from edgeless and paste to page', async ({
   page,
 }) => {