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.

Author: YousefED

packages/core/src/extensions/tiptap-extensions/KeyboardShortcuts/KeyboardShortcutsExtension.ts

@@ -136,6 +136,13 @@ export const KeyboardShortcutsExtension = Extension.create<{
               return false;
             }
 
+            const selectionAtBlockStart =
+              state.selection.from ===
+              blockInfo.blockContent.beforePos + 1;
+            if (!selectionAtBlockStart) {
+              return false;
+            }
+
             const prevBlockInfo = getPrevBlockInfo(
               state.doc,
               blockInfo.bnBlock.beforePos,
@@ -462,15 +469,22 @@ export const KeyboardShortcutsExtension = Extension.create<{
 
             return false;
           }),
-        // If the previous block is a columnList, moves the current block to
-        // the end of the last column in it.
+        // If the next block is a columnList, moves the first block from its
+        // first column to after the current block.
         () =>
           commands.command(({ state, tr, dispatch }) => {
             const blockInfo = getBlockInfoFromSelection(state);
             if (!blockInfo.isBlockContainer) {
               return false;
             }
 
+            const selectionAtBlockEnd =
+              state.selection.from ===
+              blockInfo.blockContent.afterPos - 1;
+            if (!selectionAtBlockEnd) {
+              return false;
+            }
+
             const nextBlockInfo = getNextBlockInfo(
               state.doc,
               blockInfo.bnBlock.beforePos,