feat: add Context.dev Haystack integration

[aadithyanr]

Summary

• add ContextWebSearch for ranked live-web results with optional Markdown
• add ContextFetcher for webpages and timestamped YouTube transcript Markdown
• add credit-safe ContextCrawler with explicit crawl bounds
• support sync and async execution, Haystack serialization, retries, and typed errors
• configure standalone packaging, CI, release automation, examples, and contributor docs

This follows the standalone integration route requested by the Haystack maintainers after deepset-ai/haystack-core-integrations#3871. The unpublished Answers endpoint is intentionally excluded.

Validation

• hatch run fmt-check
• hatch run test:types
• hatch run test:all — 25 passed, 6 live tests skipped without credentials
• hatch run test:cov — 96% coverage
• hatch run test:integration — 6 passed against https://api.context.dev/v1
• hatch build — source distribution and wheel built successfully
• inspected wheel contents and confirmed only intended package modules and typing marker are included