Mirror the search/route.ts pattern for /v1/web/fetch: skip rate-limited
stubs instead of letting them short-circuit auto-select, walk the
fixed-priority pool (fill-first) with a request-time fallback on
retryable/quota upstream statuses (429 always; 402/403 for
Firecrawl/Tavily/TinyFish quota-style tiers), and return a proper 429
(with Retry-After) when the whole pool is exhausted instead of a
generic 400. Explicit-provider requests never silently fall back.