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/nonce_cache/src/main.rs

@@ -160,6 +160,7 @@ async fn pumpfun_copy_trade_with_grpc(trade_info: PumpFunTradeEvent) -> AnyResul
         fixed_output_token_amount: None,
         gas_fee_strategy: gas_fee_strategy,
         simulate: false,
+        use_exact_sol_amount: None,
     };
     client.buy(buy_params).await?;