Fix unsafe type assertions in nestLists using discriminated union narrowing

[Copilot]

Replaced unsafe type assertions with proper type narrowing to satisfy typescript-eslint/no-unsafe-type-assertion.

Changes

• Check currentList.mode and newList.mode properties instead of the function parameter mode to enable TypeScript's discriminated union narrowing
• Removed three as type assertions that were flagged as unsafe

Before:

if (mode === 'html') {
  const lastListItem = currentList.children[
    currentList.children.length - 1
  ] as ToolkitPortableTextListItem
  // ...
} else {
  ;(currentList as ToolkitPortableTextDirectList).children.push(
    newList as ToolkitPortableTextDirectList,
  )
}

After:

if (currentList.mode === 'html' && newList.mode === 'html') {
  const lastListItem = currentList.children[currentList.children.length - 1]
  // TypeScript now knows currentList is ToolkitPortableTextHtmlList
  // ...
} else if (currentList.mode === 'direct' && newList.mode === 'direct') {
  currentList.children.push(newList)
  // TypeScript now knows both are ToolkitPortableTextDirectList
}

---

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