feat: add WeChat MP and X/Twitter converters

[jun7799]

Summary

• WeChatMPConverter: Convert WeChat public account articles (mp.weixin.qq.com) to Markdown. Extracts title, author, account name, content with formatting preservation. Downloads images locally to avoid link expiration. Includes anti-bot bypass mechanism.
• XTwitterConverter: Convert X/Twitter posts to Markdown via FXTwitter API. Supports:
  • Regular tweets (text + images)
  • Long-form articles (full content with headings, lists, blockquotes, code blocks, bold styles)
  • Video tweets (text + mp4 download link + thumbnail)

Both converters follow the existing DocumentConverter pattern, registered with PRIORITY_SPECIFIC_FILE_FORMAT so they are tried before the generic HtmlConverter.

Test plan

• WeChat article conversion tested with https://mp.weixin.qq.com/s/9ThAXdoL8qCX4QR7p7VVKA - title, content, 8 images all extracted correctly
• Anti-bot bypass tested - detects CAPTCHA page and retries with mobile User-Agent
• X/Twitter long article tested with https://x.com/liyue_ai/status/2042519090623922277 - 287 lines, 22 images, code blocks/quotes/lists preserved
• X/Twitter video tweet tested with https://x.com/PMbackttfuture/status/2042216207961493700 - text, video URL, thumbnail all extracted
• X/Twitter regular tweet tested with https://x.com/0xluffy_eth/status/2042849639821725887 - text, images, engagement stats all extracted
• Existing converters unaffected (no changes to shared code paths)

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