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, 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.

Author: wqxoxo

plugins/libplugin-pay.c

@@ -1702,6 +1702,11 @@ static struct command_result *payment_sendonion_success(struct command *cmd,
 							struct payment *p)
 {
 	struct out_req *req;
+	struct payment *root = payment_root(p);
+
+	if (p->invstring)
+		root->invstring_used = true;
+
 	req = jsonrpc_request_start(payment_cmd(p), "waitsendpay",
 				    payment_waitsendpay_finished,
 				    payment_waitsendpay_finished, p);
@@ -1764,8 +1769,6 @@ static struct command_result *payment_createonion_success(struct command *cmd,
 
 		if (p->description)
 			json_add_string(req->js, "description", p->description);
-
-		root->invstring_used = true;
 	}
 
 	if (p->pay_destination)