Body of PUT /providers/{layer} — save and activate one provider.

A partial write. Fields you omit keep the value already saved for this (layer, provider) pair: omit api_key to edit params without resending the key, send only the params keys you want to change, and omit fallback_enabled to leave the flag as it is. The merged result is what gets validated against the layer's schema (422 on a mismatch), so a partial write can never leave an invalid config behind.

Rows are keyed by (organization, layer, provider), so writing a different provider for the same layer starts from scratch rather than inheriting the previous provider's params. On a brand-new row fallback_enabled defaults to false.

Keys are write-only: no response ever echoes one back.