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>

src/instruction/pumpfun.rs

@@ -118,7 +118,7 @@ impl InstructionBuilder for PumpFunInstructionBuilder {
         }
 
         let mut buy_data = [0u8; 24];
-        if params.use_exact_sol_amount {
+        if params.use_exact_sol_amount.unwrap_or(true) {
             // buy_exact_sol_in(spendable_sol_in: u64, min_tokens_out: u64)
             // Spend exactly the input SOL amount, get at least min_tokens_out
             let min_tokens_out = calculate_with_slippage_sell(

src/instruction/pumpswap.rs

@@ -187,7 +187,7 @@ impl InstructionBuilder for PumpSwapInstructionBuilder {
         // Create instruction data
         let mut data = [0u8; 24];
         if quote_is_wsol_or_usdc {
-            if params.use_exact_sol_amount {
+            if params.use_exact_sol_amount.unwrap_or(true) {
                 // buy_exact_quote_in(spendable_quote_in: u64, min_base_amount_out: u64)
                 // Spend exactly the input SOL/quote amount, get at least min_base_amount_out
                 let min_base_amount_out = crate::utils::calc::common::calculate_with_slippage_sell(