[lexical-code-core] Bug Fix: Alt+Arrow no longer merges two lines when a code block line is blank

[LeSingh1]

Description

$handleShiftLines moves the selected code-block line(s) past the adjacent
line. It anchors the move on sibling — the node just past the linebreak that
separates the moving line from its neighbour — and resolves that to the
neighbour line's boundary node:

const maybeInsertionPoint =
  $isCodeHighlightNode(sibling) || $isTabNode(sibling) || $isLineBreakNode(sibling)
    ? arrowIsUp ? $getFirstCodeNodeOfLine(sibling) : $getLastCodeNodeOfLine(sibling)
    : null;

When the neighbour line is blank, sibling is itself a LineBreakNode —
the one that terminates the line on the far side of the blank line.
$getFirstCodeNodeOfLine / $getLastCodeNodeOfLine only walk over
CodeHighlightNode/TabNode, so for a LineBreakNode anchor they return it
unchanged. The moving line is then spliced against a linebreak that belongs to
another line, which appends it to that line — the two lines merge and the text
is corrupted, not merely misplaced.

With a code block containing A, a blank line, and B:

• Alt+ArrowUp on B produced AB\n\n instead of A\nB\n;
• Alt+ArrowDown on A produced \n\nAB instead of \nA\nB.

A blank LineBreakNode sibling has no node of its own to anchor to, so treat
it as its own case: drop the moving line (and its linebreak) immediately after
that sibling, which is exactly the blank line's position. That is the same
insertion for both directions. Lines whose neighbour is not blank keep taking
the existing code path unchanged.

Test plan

Two new cases in
packages/lexical-code-core/src/__tests__/unit/CodeIndentation.test.ts.

Before

$ npx vitest run packages/lexical-code-core/src/__tests__/unit/CodeIndentation.test.ts

 FAIL  |unit| .../CodeIndentation.test.ts > CodeIndentExtension > shiftLines > moves a line up past a blank line without merging it into the line above
AssertionError: expected 'AB\n\n' to be 'A\nB\n' // Object.is equality

 FAIL  |unit| .../CodeIndentation.test.ts > CodeIndentExtension > shiftLines > moves a line down past a blank line without merging it into the line below
AssertionError: expected '\n\nAB' to be '\nA\nB' // Object.is equality

- Expected
+ Received

-
- A
- B
+
+ AB

 Test Files  1 failed (1)
      Tests  2 failed | 18 passed (20)

After

$ npx vitest run packages/lexical-code-core packages/lexical-code

 Test Files  11 passed (11)
      Tests  270 passed | 1 skipped (271)

The existing code blocks can shift lines (with tab) /
code blocks can shift multiple lines (with tab) unit tests and the
can move around code block with arrow keys e2e spec use no blank lines, so
their sibling is a CodeHighlightNode and they take the unchanged path.

[vercel]

@LeSingh1 is attempting to deploy a commit to the Meta Open Source Team on Vercel.

A member of the Team first needs to authorize it.

[LeSingh1]

Consolidated into #9056 with the other PRs that share this defect, per @etrepum's note on #9027 and @mayrang's on #9035. Same fix and same tests, one review.