Fix #3087 Delete paragraph styles when there is nothing else to be deleted

[JiuqingSong]

Sometimes when paste some content that has styles on paragraph, it is hard to remove those styles. This change handles delete operation, and at the last, check if nothing is deleted, try to delete styles on current paragraph if any, so that those styles can be deleted.

[Copilot]

Pull Request Overview

This PR fixes issue #3087 by implementing a solution to delete paragraph styles when no other content can be deleted. The change addresses cases where pasted content with paragraph styles becomes difficult to remove by adding a final delete step that clears paragraph formatting when the paragraph is effectively empty.

• Adds a new deleteParagraphStyle function as the final step in the delete operation pipeline
• Updates all existing test cases to include the new delete step in their expected step arrays
• Provides comprehensive test coverage for the new paragraph style deletion logic

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 3 comments.

File	Description
keyboardDelete.ts	Adds the deleteParagraphStyle import and includes it as the final step in the delete steps array
deleteParagraphStyle.ts	Implements the core logic to delete paragraph styles when nothing else can be deleted and the paragraph is effectively empty
deleteParagraphStyleTest.ts	Provides comprehensive unit tests covering various scenarios for the new paragraph style deletion functionality
keyboardDeleteTest.ts	Updates all existing test cases to include deleteParagraphStyle in their expected delete steps arrays