_get_backend returns the stored web.backend verbatim (mapping the managed
'nous' selection to the firecrawl provider) — unknown names surface the
honest selection-naming error at dispatch instead of silently rerouting
through the credential ladder, which now runs only on never-configured
installs. _get_capability_backend no longer discards an explicit
search/extract backend when its availability probe fails. The firecrawl
client resolves strictly: 'nous' => managed gateway only (unavailable =>
selection-naming error), stored vendor => direct only (no FIRECRAWL key
=> error, never a silent managed fallback billed to Nous).
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