Self-review follow-up. check_web_api_key() had a hand-rolled 'walk all
registered providers and probe each' fallback that duplicated the registry's
own availability-filtered resolvers (get_active_search_provider /
get_active_extract_provider, backed by _resolve()) — a second resolution path
that could diverge (the hand-rolled walk ignored capability, so a search-only
custom provider was handled inconsistently). Delegate to the registry's
resolvers so there is one authority for 'is a custom provider usable'.
Also: _get_backend()'s tail walk now probes provider.is_available() directly
instead of round-tripping through _is_backend_available(provider.name), which
redundantly re-did the registry get_provider() lookup on a provider object
already in hand. Both fallback loops guard is_available() against exceptions.
Documented that _LEGACY_WEB_BACKENDS intentionally includes 'xai' (probed via
has_xai_credentials, not a registered provider) while the registry's
_LEGACY_PREFERENCE excludes it, so the two built-in sets don't silently drift.
kshitijk4poor
a9cd0e07cb