Two response fields now report only what is measured

Two values in Positions API responses on mainnet have changed. If your client reads either one, update it.

• attribution_confidence in the meta block is now null. The field is deprecated and no longer carries a number; treat it as nullable.
• In GET /v1/protocols, a protocol with no recorded health signal now reports adapter.error as "no health signal recorded for this protocol". If you match on the previous text, match on this one instead.