chore(core): pdf viewer is enabled by default (#12207)

Related to: [BS-3101](https://linear.app/affine-design/issue/BS-3101/pdf-embed-模式的选中框选-和点开看详情有比较大的问题)

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

- **New Features**
  - PDF embed preview is now enabled by default for all users, regardless of workspace sharing mode.

- **Bug Fixes**
  - Improved reliability of PDF viewer interaction in tests by updating the element used for visibility checks.
  - Enabled opening the PDF viewer peek view in readonly and sharing modes.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

packages/frontend/core/src/blocksuite/block-suite-editor/lit-adaper.tsx

@@ -74,9 +74,7 @@ const usePatchSpecs = (mode: DocMode) => {
   );
 
   const enablePDFEmbedPreview = useLiveData(
-    featureFlagService.flags.enable_pdf_embed_preview.$.map(
-      flag => !workspaceService.workspace.openOptions.isSharedMode && flag
-    )
+    featureFlagService.flags.enable_pdf_embed_preview.$
   );
 
   const patchedSpecs = useMemo(() => {