[lexical][lexical-rich-text][lexical-utils] Bug Fix: split, copy and insert keep the original node's state

[LeSingh1]

Description

Five bugs, one shape. An operation that splits, copies or inserts a block
builds the new node by hand instead of deriving it from the node it came
from, so the original's mode, alignment, format or style is silently
dropped — or the split leaves an extra empty block behind. This is the same
defect class already seen in the insertNewAfter paths that construct a
node from scratch rather than reusing the original's state.

• TextNode.splitText (packages/lexical/src/nodes/LexicalTextNode.ts)
  copied format, style, detail and NodeState onto the parts it creates but
  not __mode. Splitting a token TextNode left a token head followed by
  normal-mode tails, which are editable and can be merged into neighbouring
  text by normalization. Every part now keeps the mode of the node that was
  split; segmented nodes keep their existing behaviour of being downgraded
  to normal. ([lexical] Bug Fix: keep the text mode on every part of a split TextNode #8924)

• ParagraphNode.extractWithChild
  (packages/lexical/src/nodes/LexicalParagraphNode.ts) did not exist, so
  ParagraphNode inherited the default of false and copying a single,
  fully selected paragraph dropped the paragraph itself from both clipboard
  payloads — its alignment, indent and style had nowhere to live and the
  paste came out left aligned. HeadingNode and QuoteNode already
  override this. ParagraphNode now does too, but only when the selection
  covers the whole paragraph and that paragraph actually carries
  block-level state, so a plain paragraph still copies as the inline-only
  content existing clipboard consumers expect. ([lexical] Bug Fix: keep paragraph alignment when copying a whole paragraph #8944, fixes Bug: Text alignment is lost on copy-paste #8101)

• ParagraphNode.collapseAtStart (same file) decided a paragraph was
  "empty" from children[0] alone. A first paragraph whose first text node
  is blank but whose later children carry the content — a leading space
  followed by a bold run — was treated as empty, so Backspace at its start
  removed the whole paragraph and lost the content; a non-text child such
  as a line break was discarded the same way. It now requires every child
  to be a text node and the paragraph's whole text content to be blank.
  Blank and empty first paragraphs still collapse exactly as before. ([lexical] Bug Fix: collapseAtStart no longer discards a first paragraph that only starts blank #8975)

• $insertNodeToNearestRoot (packages/lexical-utils/src/index.ts) splits
  the enclosing blocks at the selection focus, so a focus at the end of a
  block left an empty right-hand half — a spurious empty paragraph or list
  item after the inserted node. That empty block is only wanted when the
  insertion point is at the very end of its nearest root, where it is the
  only place left for the selection to go, so a $shouldSplit now declines
  the empty 'last' split when the new $hasContentAfter helper reports
  content still following the caret. ([lexical-utils] Bug Fix: no extra paragraph when $insertNodeToNearestRoot splits at the end of a block #8976, fixes Bug: $insertNodeToNearestRoot inserts extra paragraph node if layout node is inserted at the end of text node. #5433)

• QuoteNode.insertNewAfter (packages/lexical-rich-text/src/index.ts)
  built the continuation block by hand, named its RangeSelection argument
  _ and carried across only the direction, so Enter at the end of a
  centred quote produced a left-aligned paragraph while the identical
  keystroke in a centred paragraph did not. It now carries the block format
  and style and the caret's pending text format and style, matching core
  ParagraphNode.insertNewAfter. ([lexical-rich-text] Bug Fix: Enter at the end of a quote keeps the block alignment and style #9028)

Test plan

Each fix brings its own unit tests: splitText keeps the token mode on every part in LexicalTextNode.test.tsx, the new Issue8101Repro.test.ts
and ParagraphCollapseAtStart.test.ts under packages/lexical, two new
cases in LexicalUtilsInsertNodeToNearestRoot.test.tsx, and the new
QuoteInsertNewAfter.test.ts under packages/lexical-rich-text. The
quote tests run the same helper against a paragraph and against a quote and
require them to agree, so they are pinned to core's behaviour rather than to
a hardcoded expectation. #8975 additionally adds a Playwright case to
packages/lexical-playground/__tests__/e2e/Selection.spec.mjs; that spec
needs a built playground and was not run locally, so it is covered by CI's
e2e job rather than by the output below.

Before

Source fixes reverted, new tests kept:

$ npx vitest run --project unit packages/lexical/src packages/lexical-utils packages/lexical-rich-text
 × splitText keeps the token mode on every part
 × keeps a first paragraph whose content follows a blank text node
 × keeps a first paragraph whose only other child is a line break
 × copying a whole aligned paragraph keeps the alignment in the Lexical clipboard payload
 × copying a whole aligned paragraph keeps the alignment in the HTML clipboard payload
 × copy a centered paragraph and paste it into an empty paragraph
 × keeps the block alignment and style, as ParagraphNode does
 × seeds the continuation block with the pending text format
 × insert in the end of paragraph that is followed by another block
 × insert in the end of the last nested list item

 Test Files  5 failed | 83 passed (88)
      Tests  10 failed | 1622 passed (1632)

After

$ npx vitest run --project unit packages/lexical/src packages/lexical-utils packages/lexical-rich-text

 Test Files  88 passed (88)
      Tests  1632 passed (1632)

$ npx tsc --noEmit -p .
(clean)

Supersedes #8924, #8944, #8975, #8976, #9028, consolidated per the review
feedback on #9027 and #9035.

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