fix(editor): comment style and creation issues (#13006)

#### PR Dependency Tree


* **PR #13006** 👈

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[Charcoal](https://github.com/danerwilliams/charcoal)

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

## Summary by CodeRabbit

* **Style**
* Updated embedded note content to remove background color and underline
from inline comments for a cleaner appearance.

* **Bug Fixes**
* Improved comment functionality by ensuring comments are only added to
blocks without text content.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

blocksuite/affine/shared/src/services/comment-service/block-comment-manager.ts

@@ -70,7 +70,7 @@ export class BlockCommentManager extends LifeCycleWatcher {
           }
         );
         if (!selectedBlocks) return [];
-        return selectedBlocks.map(b => b.model);
+        return selectedBlocks.map(b => b.model).filter(m => !m.text);
       });
 
     const needCommentBlocks = [