fix(ri-exchange): an unknown accept outcome is marked failed and retryable #1050

Problem

An RI exchange whose accept response was lost (common.ErrOutcomeUnknown, raised by go's single-shot accept from go#299) may already have executed on AWS, but the platform treats it as a plain failure:

• Approve flow: internal/api/handler_ri_exchange.go (~:2666) calls exchange.ExecuteExchange; at ~:2682 an ErrOutcomeUnknown-wrapped error goes to failExchange, which marks the row failed, releases the cap and leaves it retryable.
• Interactive flow (~:1921): mapAWSExchangeError maps it to a generic 500 (claimExchangeSubmit partly mitigates a resubmit).
• Scheduled flow: the exchange email (~:124-215) lists it under failures.
• ReserveRIExchange (internal/config/store_postgres.go ~:2996-3018) has no guard against reserving the same source RI again while an earlier attempt's outcome is unknown.

Whether AWS rejects a second accept for an already-exchanged source RI is not verified, so a repeat attempt may or may not be safe.

Expected

• Approve and interactive flows: detect errors.Is(err, common.ErrOutcomeUnknown), mark the row "outcome unknown, reconcile" (not failed, not retryable), and tell the operator to check the source RI and target reservation in the account.
• Scheduled flow: read ExchangeOutcome.OutcomeUnknown (being added in cloud-commitments-go#443, which keeps such outcomes in Failed) and render them separately.
• Block reserving a source RI that has an unresolved outcome-unknown attempt.

Found while planning cloud-commitments-go#443.