Save a provider for ``layer`` and make it the active one. A partial write: anything you omit keeps its saved value. Omitting ``api_key`` keeps the stored key, ``params`` keys you don't send are preserved, and omitting ``fallback_enabled`` leaves the flag alone (``false`` on a new row). The merged result — not the partial input — is validated against the layer's schema (422 on a mismatch). 404 on an unknown layer.