sendLightning: stop warning on healthy multi-path payments

Each xpay with failed_parts > 0 that ultimately settled (multi-path
retry — common, not concerning) was firing 3 WARN lines:
  "xpay returned (abnormal)"
  "xpay returned failed_parts without throwing"
  "xpay failed_parts but listpays complete — finalizing"

These are all the SAME successful payment. The presence of failed_parts
just means CLN tried multiple paths before one succeeded. Adam was
seeing noise.

Now:
- Only warn on actually-missing preimage (xpay silently failed).
- Drop the unconditional "without throwing" warn — the failed_parts
  branch handles the recovery silently in the success case.
- Drop the "listpays complete — finalizing" warn — that's the success
  path, not a concern.
- KEEP warns for the genuinely interesting cases: listpays says failed
  (we're about to reverse), listpays says pending (debit left for
  check() to reconcile), or finalize itself throws.

Net effect: a healthy multi-path payment now emits only the standard
"sendLightning outcome <id> = finalized-via-listpays-after-failed_parts"
info line instead of 3 WARN lines.

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

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