[lexical-code-core][lexical-code-prism][lexical-code-shiki] Bug Fix: code blocks keep the caret and their own attributes

[LeSingh1]

Description

Code blocks mishandle their own line and selection bookkeeping: the highlighter
restores a selection point that the re-highlight has already invalidated, the
line-shift and outdent helpers mis-measure a line that is blank or whose caret
sits at column 0, and the importer never reads back the data-theme that the
exporter writes. The user-visible result is a caret that gets dragged into (or
thrown out of) a code block whenever it re-highlights or re-indents, and a block
that loses its own attributes on an HTML round trip.

This consolidates five fixes:

• [lexical-code-prism][lexical-code-shiki] Bug Fix: don't restore a negative selection offset when a code line starts with a line break #8947 — $updateAndRetainSelection in CodeHighlighterPrism.ts and
  CodeHighlighterShiki.ts walked the code node's children with .some() and
  only decremented the running offset for text nodes, so a LineBreakNode at
  the start of a code line drove textOffset negative and the next text node
  was selected at an out-of-range offset. The walk is now an explicit loop: a
  LineBreakNode consumes one unit of offset, and when the offset lands on one
  an element point on the code node is used instead of a negative text point.

• [lexical-code-prism][lexical-code-shiki] Bug Fix: don't drag the caret into a code block while highlighting #8972 — the same $updateAndRetainSelection retained the selection even
  when the anchor was not inside the code node being highlighted, dragging the
  caret into a code block after e.g. a markdown import. It now falls back to a
  plain updateFn() unless the anchor is the code node or a descendant of it.

• [lexical-code-core] Bug Fix: Alt+Arrow no longer merges two lines when a code block line is blank #8986 — $handleShiftLines in CodeIndentation.ts accepted a
  LineBreakNode sibling as an insertion-point candidate, but that linebreak
  belongs to a different line, so Alt+Arrow across a blank line spliced the
  moving line into the line beyond it and merged the two. A blank adjacent line
  is now detected up front and the range is re-inserted directly after the
  sibling linebreak, in both directions.

• [lexical-code-core] Bug Fix: restore the code block theme from data-theme on import #8988 — $convertPreElement in CodeNode.ts and the pre /
  multiline-code import rules in CodeImportExtension.ts read data-language
  but not data-theme, so exportDOM wrote a theme that importDOM silently
  dropped. Both now pass data-theme through to $createCodeNode.

• [lexical-code-core] Bug Fix: Shift+Tab outdents a code line when the caret is at column 0 #8992 — $getCodeLines drops a trailing line when the selection ends
  exactly at its start, which for a collapsed caret is always true, so Shift+Tab
  at column 0 never reached the outdent loop and silently did nothing. A new
  $outdentLineAtCaret in CodeIndentation.ts resolves the line from the
  anchor and applies the same rule as that loop: strip a leading TabNode, or
  tabSize leading spaces when the extension is configured for them.

Test plan

New unit tests per fix: selection-offset retention for both Prism and Shiki,
caret placement after a markdown import that ends in a code block, Alt+Arrow
across a blank line, data-theme import plus an exportDOM/importDOM round
trip, and Shift+Tab at column 0 (tab indent, space indent, a non-zero column,
and an unindented no-op control).

Before

Source fixes reverted, new tests kept:

$ npx vitest run --project unit packages/lexical-code-core packages/lexical-code-prism packages/lexical-code-shiki

 FAIL  |unit| packages/lexical-code-prism/src/__tests__/unit/CodeHighlighterPrismRetainSelection.test.ts > CodeHighlighterPrism $updateAndRetainSelection > retains a non-negative offset for code content "\nfoo" (#8943)
 FAIL  |unit| packages/lexical-code-prism/src/__tests__/unit/CodeHighlighterPrismRetainSelection.test.ts > CodeHighlighterPrism $updateAndRetainSelection > retains a non-negative offset for code content "\n\nfoo" (#8943)
 FAIL  |unit| packages/lexical-code-prism/src/__tests__/unit/CodePrismRetainSelection.test.ts > Prism highlighting only retains a selection it owns (#6305) > importing markdown that ends in a code block leaves the caret at the document start
 FAIL  |unit| packages/lexical-code-prism/src/__tests__/unit/CodePrismRetainSelection.test.ts > Prism highlighting only retains a selection it owns (#6305) > the caret is not dragged into the last of several imported code blocks
 FAIL  |unit| packages/lexical-code-shiki/src/__tests__/unit/CodeShikiRetainSelection.test.ts > CodeHighlighterShiki $updateAndRetainSelection > retains a non-negative offset for code content "\nfoo" (#8943)
 FAIL  |unit| packages/lexical-code-shiki/src/__tests__/unit/CodeShikiRetainSelection.test.ts > CodeHighlighterShiki $updateAndRetainSelection > retains a non-negative offset for code content "\n\nfoo" (#8943)
 FAIL  |unit| packages/lexical-code-shiki/src/__tests__/unit/CodeShikiRetainSelection.test.ts > Shiki highlighting only retains a selection it owns (#6305) > importing markdown that ends in a code block leaves the caret at the document start
 FAIL  |unit| packages/lexical-code-shiki/src/__tests__/unit/CodeShikiRetainSelection.test.ts > Shiki highlighting only retains a selection it owns (#6305) > the caret is not dragged into the last of several imported code blocks
 FAIL  |unit| packages/lexical-code-core/src/__tests__/unit/CodeImportExtension.test.ts > CodeImportExtension > <pre data-theme="poimandres"> restores the theme
 FAIL  |unit| packages/lexical-code-core/src/__tests__/unit/CodeImportExtension.test.ts > CodeImportExtension > multi-line <code data-theme> restores the theme
 FAIL  |unit| packages/lexical-code-core/src/__tests__/unit/CodeIndentation.test.ts > CodeIndentExtension > shiftLines > moves a line up past a blank line without merging it into the line above
 FAIL  |unit| packages/lexical-code-core/src/__tests__/unit/CodeIndentation.test.ts > CodeIndentExtension > shiftLines > moves a line down past a blank line without merging it into the line below
 FAIL  |unit| packages/lexical-code-core/src/__tests__/unit/CodeNode.test.ts > CodeNode > round-trips the theme through exportDOM/importDOM
 FAIL  |unit| packages/lexical-code-core/src/__tests__/unit/CodeOutdentCollapsedAtLineStart.test.ts > OUTDENT_CONTENT_COMMAND at the start of a code line > outdents when the caret is collapsed at column 0 (on the TabNode)
 FAIL  |unit| packages/lexical-code-core/src/__tests__/unit/CodeOutdentCollapsedAtLineStart.test.ts > OUTDENT_CONTENT_COMMAND at the start of a code line > outdents when the caret is collapsed at column 0 of the code text
 FAIL  |unit| packages/lexical-code-core/src/__tests__/unit/CodeOutdentCollapsedAtLineStart.test.ts > OUTDENT_CONTENT_COMMAND at the start of a code line > strips a space indent from column 0 when tabSize is configured

 Test Files  7 failed | 7 passed (14)
      Tests  16 failed | 218 passed (234)

After

$ npx vitest run --project unit packages/lexical-code-core packages/lexical-code-prism packages/lexical-code-shiki

 Test Files  14 passed (14)
      Tests  234 passed (234)

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

Supersedes #8947, #8972, #8986, #8988, #8992, consolidated per the review
feedback on #9027 and #9035.

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