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>

README.md

@@ -178,6 +178,7 @@ let buy_params = sol_trade_sdk::TradeBuyParams {
   fixed_output_token_amount: None,  // Optional: specify exact output amount
   gas_fee_strategy: gas_fee_strategy.clone(),  // Gas fee strategy configuration
   simulate: false,  // Set to true for simulation only
+  use_exact_sol_amount: None,  // Use exact SOL input for PumpFun/PumpSwap (defaults to true)
 };
 ```