pay: MPP fails over a single channel

### Issue and Steps to Reproduce
Experiencing a similar issue to #5250 trying to pay simple $5 invoice to a BTCPayServer of Lunanode.
My CLN node has a single channel to a routing node currently still the pay plugin wants to send with MPP all the time.
There is no other way, but to put `disable-mpp` in the config.
It seems this will need to be done for all nodes with only one channel otherwise CLN cannot pay anything.
The connected routing node can pay the invoice every time and also CLN can if `disable-mpp` is set.

Telegram entry: https://t.me/lightningd/12354

### `getinfo` output
```
{
"id": "020fc7e62c9956a34bcdba309a4809ccfd7adcae4b8b644cac81b4dc58d43a2479",
"alias": "LNroadz",
"color": "020fc7",
"num_peers": 1,
"num_pending_channels": 0,
"num_active_channels": 1,
"num_inactive_channels": 0,
"address": [
{
"type": "torv3",
"address": "dt2xgzg4wg5zksqlzxmtqx3yx4dnic7hlysvycfvunbrgoatfzsnosyd.onion",
"port": 9736
}
],
"binding": [
{
"type": "ipv4",
"address": "127.0.0.1",
"port": 9736
}
],
"version": "v0.11.0.1",
"blockheight": 736305,
"network": "bitcoin",
"msatoshi_fees_collected": 805680,
"fees_collected_msat": "805680msat",
"lightning-dir": "/home/bitcoin/.lightning/bitcoin",
"our_features": {
"init": "88282269a2",
"node": "800088282269a2",
"channel": "",
"invoice": "02000020024100"
}
}
```

payプラグインのMPPパスから始め、getinfoに表示される単一のchannelを持つnodeで$5のinvoiceの支払いを再現します。デフォルトの動作とdisable-mpp設定を比較します。この回避策なしで支払いが成功し、単一channelのケースがリグレッションに対してカバーされれば完了です。

索引モデルが issue の本文から書いたものです。