Adds failing tests that document setNodeIndentFromDOM's two failure modes:
parseInt cannot read the reconciler's `calc(N * var(--lexical-indent-base-value, 40px))`
inline style, and the hardcoded /40 divisor ignores a custom
`--lexical-indent-base-value`. The literal `${indent * 40}px` round-trip
that exportDOM produces still passes, so this test is added to document
the gap.

- setUpEditor now declares RichTextExtension as a dependency of the test
  extension and takes the `$initialEditorState` as an argument, rather
  than composing it as a sibling root extension at every call site.
- $importHtml is a pure `$` function and must be invoked inside
  `editor.update`, instead of opening its own update transaction.
- Replace `ReturnType<typeof setUpEditor>` with an imported
  `LexicalEditor` type.

- $importHtml uses $getEditor and pastes with $selectAll(), so it
  replaces whatever was in the editor instead of requiring callers to
  root.clear() first.
- Drop the editor argument from $importHtml — it's a $ function.
- Replace `getFirstChildOrThrow<ParagraphNode>()` with
  `assert($isParagraphNode(para), ...)` so the type narrowing also
  doubles as an assertion that imports produced a ParagraphNode.

…repro

The tests only exercise the model and the HTML import/export paths, so
the editor doesn't need to be mounted.