Fixes#6978 where bolt11 annotations were lost when sendonion failed early and payment was retried.
When sendonion RPC fails before saving payment to database, invstring_used flag would remain true, causing retry attempts to omit bolt11 parameter. Successful retries would then save to DB without bolt11 annotation.
Move invstring_used flag setting from payment_createonion_success to payment_sendonion_success. This ensures the flag is only set after sendonion actually succeeds. The bolt11 will be sent with every sendonion attempt until the first successful one, accepting the minor redundancy for cleaner state management.
Changelog-Fixed: Plugins: `listpays` can be missing the bolt11 information in some cases where `pay` is used.
Any file in this directory which is executable and whose name only
consists of alphanumeric characters, space, '.', '-' or '_' will be
automatically loaded when lightningd starts (unless suppressed with
commandline options).
b906507968