Make use_exact_sol_amount optional with true default for PumpFun/PumpSwap

- Change use_exact_sol_amount from bool to Option<bool>
- Default to true (via unwrap_or) for PumpFun and PumpSwap DEXes
- Option is ignored for other DEXes (Raydium, Bonk, Meteora)
- Update all examples to use None (which defaults to true for applicable DEXes)
- Update README documentation

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

examples/middleware_system/src/main.rs

@@ -102,6 +102,7 @@ async fn test_middleware() -> AnyResult<()> {
         fixed_output_token_amount: None,
         gas_fee_strategy: gas_fee_strategy,
         simulate: false,
+        use_exact_sol_amount: None,
     };
     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");