sendLightning/sendKeysend: instrument exit paths, tighten LEAKED DEBIT guard

Port of master 51107ed4 + fe767002 + cb0f1962 (combined) to the staging
stack. Track an outcome string through every exit path, handle the
xpay failed_parts branch explicitly (verify with listpays then finalize/
reverse/leave-pending), and dump abnormal xpay/keysend response shapes.
Leak guard re-reads the db record before firing: if ref is persisted the
payment is settled regardless of which path threw (finalize may set ref
then throw on the later tbRefund), so only a genuinely missing ref logs
"LEAKED DEBIT". Uses warn (no warnThrottled on this stack) and keeps the
existing retry_for: 20.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

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