Fix 298633

[JiuqingSong]

Bug 298633: Ensure bullets and sub-bullets with blank lines in between are not converted to additional bullets by Win32

When press BACKSPACE at beginning of a paragraph under list, we first remove its list number or bullet, then next time it will outdent to the parent level. To remove the list number or bullet, currently we are setting LI display to be "block" so browser will not render it as a list item. However, this behavior is not supported by Win32 Outlook, so we are doing some workaround to merge this kind of list item into previous LI. But if the previous node is not LI, this becomes a problem. For example:

<ol><ol><li>test1</li></ol><li>test2</li></ol><!--{"type":"range","start":[0,1,0,0],"end":[0,1,0,0],"isReverted":false,"isDarkMode":false}-->

This change does the following fixes:

1. When focus is not at beginning of a paragraph, do not let deleteList() handle it. It should be handled by other normal cases
2. When focus is at beginning of paragraph under a list, if the list already has displayForDummyItem, outdent it (legacy behavior)
3. When focus is at beginning of paragraph which is the first block of a list litem, and previous block is also a list item, merge this paragraph and all followed paragraphs into previous list item
4. When focused paragraph is not the first block, split current list item into two parts, make current paragraph and all followed paragraph directly under parent block if list item has only one level, or outdent.
5. Call mutateBlock to mutate the list item to fix a bug when list cache is enabled

After this change, for the case shown above, pressing BACKSPACE will cause current list item to be unlisted since it can't be merged into previous list item (because list level counts are different)

[Copilot]

Pull request overview

This PR fixes Bug 298633 related to handling list item deletion when pressing BACKSPACE at the beginning of a paragraph. The fix ensures proper behavior when dealing with nested lists and prevents incorrect merging when list level counts differ.

• Refactored the deleteList function to handle multiple scenarios: legacy dummy items, merging with previous list items, and splitting list items
• Added validation to ensure deletion only occurs at the beginning of paragraphs
• Updated test expectations to reflect the new behavior and added comprehensive test cases for edge scenarios

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
packages/roosterjs-content-model-plugins/lib/edit/deleteSteps/deleteList.ts	Refactored list deletion logic to properly handle merging list items with same level counts, splitting list items, and removing legacy dummy items
packages/roosterjs-content-model-plugins/test/edit/deleteSteps/deleteListTest.ts	Updated existing test expectations and added new test cases covering displayForDummyItem behavior, merging with previous list items, different level counts, and paragraph splitting scenarios

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