fix(core): backspace mid-text next to columnList moves block BLO-1126 (#2629)

* fix(core): add missing selectionAtBlockStart guard in backspace columnList handler

The "move to end of prev columnList" backspace handler was missing a
check for whether the cursor is at the start of the block. This caused
mid-text backspace next to a columnList to move the entire block into
the column instead of deleting a character.

* fix(core): add missing selectionAtBlockEnd guard in delete columnList handler

Same bug as the backspace handler — the "move first block from next
columnList" delete handler was missing a selectionAtBlockEnd check,
causing mid-text delete next to a columnList to incorrectly move blocks.

Also adds 3 delete tests mirroring the backspace tests, and marks
the mid-text tests with TODOs for vitest browser mode migration.