Two ABC additions to cover the surface area of the remaining four
providers (exa, parallel, tavily, firecrawl) which were untouched by the
initial spike:
1. supports_crawl() + crawl() — Tavily natively crawls a seed URL via
its /crawl endpoint. Exposing supports_crawl=True lets the crawl
tool's dispatcher route to Tavily when configured, falling back to
the auxiliary-model summarization path otherwise. Firecrawl could
add this in a follow-up (the SDK supports it; we just don't surface
it as a tool today).
2. Async-or-sync extract() — Parallel's SDK is natively async
(AsyncParallel.beta.extract); Exa and Tavily are sync; Firecrawl is
sync but called inside asyncio.to_thread() with a 60s timeout. The
ABC docstring now permits either shape: implementations declare
their own sync/async signature and the dispatcher uses
inspect.iscoroutinefunction to detect and await.
Also adds get_active_crawl_provider() to web_search_registry mirroring
the search/extract resolvers, with web.crawl_backend as the explicit
override config key.
No behavior change on its own — these are scaffolds for the four
remaining provider migrations.
