fix(wallet): persist payment_proof when transitioning melt quote state

The wallet's update_melt_quote_state() was receiving the payment_proof
parameter and forwarding it to add_transaction_for_pending_melt() but never
writing it to the persisted quote record. As a result, the proof (lightning
preimage / onchain outpoint) was never visible on the quote itself after a
successful melt, only in the transaction log.

Assign payment_proof to quote.payment_proof before persistence, and carry it
through the ConcurrentUpdate retry path.

Author: thesimplekid

crates/cdk/src/wallet/melt/mod.rs

@@ -964,6 +964,7 @@ impl Wallet {
         }
 
         quote.state = new_state;
+        quote.payment_proof = payment_proof;
 
         match self.localstore.add_melt_quote(quote.clone()).await {
             Ok(_) => Ok(()),
@@ -980,6 +981,7 @@ impl Wallet {
                         .ok_or(Error::UnknownQuote)?;
 
                     fresh_quote.state = new_state;
+                    fresh_quote.payment_proof = quote.payment_proof.clone();
 
                     match self.localstore.add_melt_quote(fresh_quote.clone()).await {
                         Ok(_) => (),