Rename use_exact_in_instruction to use_exact_sol_amount

More intuitive name that better describes what the flag does.

🤖 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_in_instruction {
+        if params.use_exact_sol_amount {
             // 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(