Preserve rendered formulas when syncing Toutiao

[sean0407]

Summary / 概述

Fixes Toutiao sync dropping LaTeX formula markup by using the editor's rendered HTML clipboard content when it is available. The existing body/Markdown fallback remains in place.

Related Issue / 关联 Issue

Closes #254

Type of Change / 更改类型

• Bug fix / 修复 Bug

Changes Made / 更改内容

• Include Toutiao in the existing rich-HTML clipboard capture path.
• Pass the rendered HTML into the Toutiao sync handler and insert it into the ProseMirror editor.
• Keep the current line-based fallback for cases without clipboard HTML.

Implementation Details / 实现细节

The Toutiao handler previously rebuilt the body from newline-separated text, which bypassed the rendered formula markup. It now prefers the same rendered HTML clipboard content already used by other platform handlers.

Testing / 测试

• npm test — passed (8 tests).
• npx --yes prettier@3.9.10 --check apps/extension/src/inject.js packages/core/src/platforms/toutiao.js tests/toutiao.test.mjs — passed.
• The new regression tests fail against the original handler because it inserts the raw $$x = y$$ text and does not forward clipboard HTML.

Testing Checklist / 测試清單

• I have tested this code locally / 我已在本地測試此程式碼
• All existing tests pass / 所有現有測試通過
• I have added tests for new functionality / 我已為新功能新增測試
• I have tested on the affected platform(s) / 我已在受影響的平台測試
• I have verified the changes work in the target browser(s) / 我已驗證更改在目標瀏覽器有效

Manual Testing Steps / 手動測試步驟

1. Prepare an article containing inline and block LaTeX formulas in the Markdown editor.
2. Select 今日头条 as a sync destination and start publishing.
3. Confirm the Toutiao draft retains the rendered formulas.

Merge Danger

The change uses rendered clipboard HTML when present; the existing body/Markdown fallback remains available otherwise.