Add Keenable (https://keenable.ai) as a web_search backend, modeled on
the existing httpx-based providers. Unlike key-gated providers, Keenable
has a no-login free tier, so it resolves to itself even without an API
key instead of falling back to DuckDuckGo; the X-API-Key header is only
sent when a key is configured (config api_key or KEENABLE_API_KEY env).
Maps result snippet (falling back to description) into the shared
content field. Covered by tests for keyed/anonymous search and the
no-fallback concurrency behavior.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>