[lexical-code-core] Bug Fix: restore the code block theme from data-theme on import

[LeSingh1]

Description

CodeNode.exportDOM writes the theme next to the language:

const language = this.getLanguage();
if (language) {
  element.setAttribute(LANGUAGE_DATA_ATTRIBUTE, language);
  ...
}
const theme = this.getTheme();
if (theme) {
  element.setAttribute(THEME_DATA_ATTRIBUTE, theme);   // 'data-theme'
}

but nothing ever read data-theme back. $convertPreElement (used for both
<pre> and multi-line <code> on the legacy path) and the PreRule /
MultilineCodeRule import rules on the @lexical/html path all built the node
with $createCodeNode(el.getAttribute(LANGUAGE_DATA_ATTRIBUTE)) and dropped the
theme. A repo-wide grep for data-theme found only writers, no readers.

CodeNode has had a theme since the two-argument $createCodeNode(language, theme); it is part of SerializedCodeNode and survives JSON round-trips, and
@lexical/code-shiki sets it on real documents. Only the HTML round trip lost
it, so copy/pasting a code block between editors — or
$generateHtmlFromNodes -> $generateNodesFromDOM — silently reverted the
block to the default theme while keeping its language.

Read data-theme in all three converters, mirroring how data-language is
already handled. The heuristic converters for pasted third-party code
(<div style="font-family: monospace">, GitHub code tables) are untouched —
they never carry a Lexical data-theme.

Test plan

Two new cases in
packages/lexical-code-core/src/__tests__/unit/CodeImportExtension.test.ts
(the @lexical/html rule pipeline) and one export -> import round trip in
packages/lexical-code-core/src/__tests__/unit/CodeNode.test.ts (the legacy
importDOM path). The round trip asserts the export side too, so it pins both
halves of the contract.

Before

Verified by restoring CodeNode.ts and CodeImportExtension.ts to their
pre-fix contents with the new tests in place:

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

⎯⎯⎯⎯⎯⎯⎯ Failed Tests 3 ⎯⎯⎯⎯⎯⎯⎯
 FAIL  |unit| .../CodeImportExtension.test.ts > CodeImportExtension > <pre data-theme="poimandres"> restores the theme
 FAIL  |unit| .../CodeImportExtension.test.ts > CodeImportExtension > multi-line <code data-theme> restores the theme
 FAIL  |unit| .../CodeNode.test.ts > CodeNode > round-trips the theme through exportDOM/importDOM

AssertionError: expected undefined to be 'poimandres' // Object.is equality

- Expected:
"poimandres"

+ Received:
undefined

      Tests  3 failed | 14 passed (17)

After

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

 Test Files  11 passed (11)
      Tests  271 passed | 1 skipped (272)

[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.