feat(x-oembed): add Twitter thread extraction and API fallback support

[kvh03]

Enhance XOembedExtractor to fetch complete Twitter/X threads when available, rather than limiting extraction to the original tweet. Additionally, implement a v1 API fallback to handle rate limiting and service outages gracefully.

• Thread Extraction: Integrated FxTwitter v2 Thread API to retrieve full thread conversations.
• Fallback Cascade: Implemented a reliable API fallback sequence:
  1. FxTwitter v2: Thread/Article API.
  2. FxTwitter v1: Single-tweet fallback.
  3. oEmbed: Twitter's official oEmbed API as the final safeguard.
• Consistency: Standardized HTML output for threads (using <hr> separators) to align with existing conversation extractor patterns.
• Testing: Added new regression test suites in tests/x-oembed-surrogates.test.ts covering successful thread rendering, rate-limit (429) cascades, and complete API failure (500) scenarios.