Improve wallet example with memory store

- Replaced unwrap() calls with proper error handling using `?` operator.
- Added comments for better code understanding.
- Introduced a timeout mechanism to prevent infinite loops when checking the quote state.
- Ensured the main function returns a Result type for graceful error handling.
- Added descriptive comments to explain the functionality of each section in the code.

crates/cdk/examples/wallet.rs

@@ -1,8 +1,5 @@
-//! Wallet example with memory store
-
 use std::sync::Arc;
 use std::time::Duration;
-
 use cdk::amount::SplitTarget;
 use cdk::cdk_database::WalletMemoryDatabase;
 use cdk::nuts::{CurrencyUnit, MintQuoteState};
@@ -13,40 +10,55 @@ use rand::Rng;
 use tokio::time::sleep;
 
 #[tokio::main]
-async fn main() {
+async fn main() -> Result<(), Box<dyn std::error::Error>> {
+    // Generate a random seed for the wallet
     let seed = rand::thread_rng().gen::<[u8; 32]>();
 
+    // Mint URL and currency unit
     let mint_url = "https://testnut.cashu.space";
     let unit = CurrencyUnit::Sat;
     let amount = Amount::from(10);
 
+    // Initialize the memory store
     let localstore = WalletMemoryDatabase::default();
 
-    let wallet = Wallet::new(mint_url, unit, Arc::new(localstore), &seed, None).unwrap();
+    // Create a new wallet
+    let wallet = Wallet::new(mint_url, unit, Arc::new(localstore), &seed, None)?;
 
-    let quote = wallet.mint_quote(amount, None).await.unwrap();
+    // Request a mint quote from the wallet
+    let quote = wallet.mint_quote(amount, None).await?;
 
     println!("Pay request: {}", quote.request);
 
+    // Check the quote state in a loop with a timeout
+    let timeout = Duration::from_secs(60); // Set a timeout duration
+    let start = std::time::Instant::now();
+
     loop {
-        let status = wallet.mint_quote_state(&quote.id).await.unwrap();
+        let status = wallet.mint_quote_state(&quote.id).await?;
 
         if status.state == MintQuoteState::Paid {
             break;
         }
 
+        if start.elapsed() >= timeout {
+            eprintln!("Timeout while waiting for mint quote to be paid");
+            return Err("Timeout while waiting for mint quote to be paid".into());
+        }
+
         println!("Quote state: {}", status.state);
 
         sleep(Duration::from_secs(5)).await;
     }
 
+    // Mint the received amount
     let receive_amount = wallet
         .mint(&quote.id, SplitTarget::default(), None)
-        .await
-        .unwrap();
+        .await?;
 
     println!("Minted {}", receive_amount);
 
+    // Send the token
     let token = wallet
         .send(
             amount,
@@ -56,8 +68,9 @@ async fn main() {
             &SendKind::default(),
             false,
         )
-        .await
-        .unwrap();
+        .await?;
 
     println!("{}", token);
+
+    Ok(())
 }