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/lib.rs

@@ -188,9 +188,10 @@ pub struct TradeBuyParams {
     /// Whether to simulate the transaction instead of executing it
     pub simulate: bool,
     /// Use exact SOL amount instructions (buy_exact_sol_in for PumpFun, buy_exact_quote_in for PumpSwap).
-    /// When true, the exact SOL/quote amount is spent and slippage is applied to output tokens.
-    /// When false (default), uses regular buy instruction where slippage is applied to SOL/quote input.
-    pub use_exact_sol_amount: bool,
+    /// When Some(true) or None (default), the exact SOL/quote amount is spent and slippage is applied to output tokens.
+    /// When Some(false), uses regular buy instruction where slippage is applied to SOL/quote input.
+    /// This option only applies to PumpFun and PumpSwap DEXes; it is ignored for other DEXes.
+    pub use_exact_sol_amount: Option<bool>,
 }
 
 /// Parameters for executing sell orders across different DEX protocols
@@ -622,7 +623,7 @@ impl TradingClient {
             fixed_output_amount: params.fixed_output_token_amount,
             gas_fee_strategy: params.gas_fee_strategy,
             simulate: params.simulate,
-            use_exact_sol_amount: false,
+            use_exact_sol_amount: None,
         };
 
         // Validate protocol params