fix(exchange): surface an unknown accept outcome as a typed result, not a plain failure #443

Follow-up from the independent review of #442 (RI exchange accept sent once, halt on unknown outcome).

Problem

When the accept outcome is unknown, processRecommendation puts the recommendation in result.Failed (pkg/exchange/auto.go:266-269, 552-559). RunAutoExchange returns a nil error. The only signals of "unknown" are the error text "purchase outcome unknown" and the record staying processing.

providers/aws/ladder/reshape.go:96-110 (summarizeExchangeResult) likely counts it as a plain failure. Consumers, including the platform, can only tell "unknown, do not retry, reconcile" apart from "failed, safe to retry" by string-matching or via GetStaleProcessingExchanges.

Suggested fix

Add a typed field or bucket (for example result.Unknown, or an OutcomeUnknown flag on the per-rec result carrying errors.Is(err, common.ErrOutcomeUnknown)). Surface it in summarizeExchangeResult and in the platform's handling, so an unknown exchange is never auto-retried or shown as an ordinary failure.

Related: the duplicated classifier in pkg/exchange/outcome.go and providers/aws/internal/purchasecfg/outcome.go (the comments cross-reference each other). A shared helper reachable from both modules would remove the drift risk.