Function should_suggest_tempo_fund 

fn should_suggest_tempo_fund(status: StatusCode, body: &[u8]) -> bool

Returns true iff a 402 response carries a structured insufficient-balance problem (RFC 9457 PaymentErrorDetails).

We deliberately do not match on free-text body content or on generic verification-failed problem types, as those have many non-funding causes (bad signature, replay, expired challenge, clock skew, key provisioning, malformed auth, …).