BIP-78: fix reference sender implementation pseudocode #2096

The reference sender pseudocode diverged from the canonical BTCPayServer implementation and contained undefined variables, missing fee rate initialization, and an incorrect upper bound on the additional fee contribution, which could mislead implementers copying it as-is. https://github.com/btcpayserver/BTCPayServer.BIP78/blob/master/BIP78.Sender/PayjoinClient.cs

Given your profile, I assume that you are not actually implementing BIP 78, but that you rather pointed an LLM at a random file in this repository to farm a commit. If an LLM can trivially fix this pseudocode, it’s also not an impediment to actual implementers. Please stop creating busywork for us.