[lexical-list] Bug Fix: indenting between two nested lists no longer converts the second list's type - #9118

Description

Pressing Tab on a list item between two nested sublists of different types silently rewrites the second sublist. Tab routes INDENT_CONTENT_COMMAND to ListItemNode.setIndent, which calls $handleIndent. The item indents into the sublist above it, and the sublist below is emptied into it, so a nested <ol> and every item in it come back as bullets. A check sublist loses more: updateChildrenListItemValue clears checked outside a check list.

The caret is on middle, between a nested bullet sublist and a nested numbered sublist. After Tab on main the numbered list is gone and its items are bullets:

list (bullet)
  listitem > list (bullet)
    listitem "bullet child"
    listitem "middle"
    listitem "number one"
    listitem "number two"

After Tab with this change, the item still indents and the numbered list keeps its type and children:

list (bullet)
  listitem > list (bullet)
    listitem "bullet child"
    listitem "middle"
  listitem > list (number)
    listitem "number one"
    listitem "number two"

mergeNextSiblingListIfSameType has long stated this rule at the top level: "<ul> will merge with <ul>, but NOT <ul> with <ol>." #9050 carried it to two nested merge sites, mergeLists and ListItemNode.remove, the latter handling this very shape and joining the surrounding sublists only when getListType() matches. The branch of $handleIndent with a nested sublist on both sides merges the same way and was missed.

This change guards that merge on the same getListType() equality, so a sublist of another type stays where it is instead of being appended and removed.

Test plan

Three tests in the $handleIndent block of packages/lexical-list/src/__tests__/unit/formatList.test.ts cover bullet then number, number then bullet, and bullet then check, the last asserting checked survives. A fourth pins the same type case.

Before

The three new tests against the previous formatList.ts:

 × does not merge the surrounding sublists of a different listType
 × does not merge the surrounding sublists of a different listType, the other way round
 × keeps the checked state of a following check sublist
AssertionError: expected 'bullet' to be 'number' // Object.is equality
      Tests  3 failed | 23 passed (26)

After

26 passed. All 15 unit files in packages/lexical-list pass, 146 tests, plus LexicalListPlugin.test.tsx. eslint and prettier are clean. Browser and e2e not run.