sendLightning/sendKeysend: instrument exit paths to surface stuck-debit cases

Diagnosing the stuck "optimistic" state seen on payment 30976bc9 (joho33,
2026-05-19): confirmed=true with no preimage, not in the pending set, so
check() couldn't reconcile. Most likely path: ln.xpay returned without
throwing but with failed_parts > 0; the previous `if (!r.failed_parts)`
guard skipped finalize, no catch ran, and the debit was left stranded.

Adds:
- Log the full xpay/keysend response shape (status, failed_parts, parts,
  amount_sent_msat, has_preimage) so we can see what CLN actually returned.
- Handle the failed_parts branch explicitly — verify with listpays and
  finalize, reverse, or leave pending based on actual outcome.
- Track an `outcome` string through every branch and log it before
  returning or throwing.
- "LEAKED DEBIT" err line at the function-end if we return without a clean
  finalize — that's the exact stuck state we want zero of. grep `LEAKED
  DEBIT` in app logs to catch any future occurrences immediately.

Same instrumentation pattern applied to sendKeysend.

Co-Authored-By: Claude Opus 4.7 (1M context) <[email protected]>

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