fix: Copy selection starting in nested block

[matthewlipski]

It's possible to select & copy nested blocks without content from the parent block being selected. ProseMirror still tries to preserve the nesting structure in this case, but because content from the parent block is not selected, it basically violates the schema when creating the Fragment for the selected content. I documented this pretty extensively in the code so recommend checking that out for more details.

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[YousefED]

Nice, and clear comments. Should we specifically also test the use-case from the bug report, which is about list structures? Or does the fact that it's a list not really matter?

[matthewlipski]

Nice, and clear comments. Should we specifically also test the use-case from the bug report, which is about list structures? Or does the fact that it's a list not really matter?

Nah the list items are totally irrelevant here, I think they got specifically reported just because nesting list items is a much more common use case than nesting other types of blocks.