Fix BOLT11 annotation loss after sendonion failure

Fixes #6978 where bolt11 annotations were lost when sendonion failed
early and payment was retried.

When sendonion RPC fails before saving payment to database, the
invstring_used flag remained true, causing retry attempts to omit
the bolt11 parameter. Successful retries would then save to DB
without bolt11 annotation.

The bug is visible in payment_createonion_success() (line 1772):
flag is set before sendonion RPC completes. If sendonion fails
immediately (e.g., no peer connection), flag remains true and
retries omit bolt11.

Fix: Reset invstring_used=false in payment_rpc_failure() when
method is "sendonion", ensuring retry attempts include bolt11.

Author: wqxoxo

plugins/libplugin-pay.c

@@ -221,6 +221,11 @@ static struct command_result *payment_rpc_failure(struct command *cmd,
 						  const jsmntok_t *toks,
 						  struct payment *p)
 {
+	struct payment *root = payment_root(p);
+
+	if (streq(method, "sendonion"))
+		root->invstring_used = false;
+
 	return payment_fail(p,
 			    "Failing a partial payment due to a failed RPC call %s: %.*s",
 			    method, toks->end - toks->start, buffer + toks->start);