refactor: Update GasFeeStrategy to instance-based API and update docs

examples/middleware_system/src/main.rs

@@ -64,8 +64,6 @@ async fn create_solana_trade_client() -> AnyResult<SolanaTrade> {
     let swqos_configs: Vec<SwqosConfig> = vec![SwqosConfig::Default(rpc_url.clone())];
     let trade_config = TradeConfig::new(rpc_url, swqos_configs, commitment);
     let solana_trade = SolanaTrade::new(Arc::new(payer), trade_config).await;
-    // set global strategy
-    sol_trade_sdk::common::GasFeeStrategy::set_global_fee_strategy(150000, 500000, 0.001, 0.001);
     println!("✅ SolanaTrade client initialized successfully!");
     Ok(solana_trade)
 }
@@ -82,6 +80,9 @@ async fn test_middleware() -> AnyResult<()> {
     let recent_blockhash = client.rpc.get_latest_blockhash().await?;
     let pool_address = Pubkey::from_str("539m4mVWt6iduB6W8rDGPMarzNCMesuqY5eUTiiYHAgR")?;
 
+    let gas_fee_strategy = sol_trade_sdk::common::GasFeeStrategy::new();
+    gas_fee_strategy.set_global_fee_strategy(150000, 500000, 0.001, 0.001);
+
     let buy_params = sol_trade_sdk::TradeBuyParams {
         dex_type: DexType::PumpSwap,
         input_token_type: TradeTokenType::WSOL,
@@ -100,6 +101,7 @@ async fn test_middleware() -> AnyResult<()> {
         open_seed_optimize: false,
         durable_nonce: None,
         fixed_output_token_amount: None,
+        gas_fee_strategy: gas_fee_strategy,
     };
     client.buy(buy_params).await?;
     println!("tip: This transaction will not succeed because we're using a test account. You can modify the code to initialize the payer with your own private key");