Fix PumpFun V2 buyback fee mutability

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "sol-trade-sdk"
-version = "4.0.11"
+version = "4.0.12"
 edition = "2021"
 authors = [
     "William <byteblock6@gmail.com>",

src/instruction/pumpfun.rs

@@ -515,7 +515,7 @@ fn build_buy_v2(params: &SwapParams) -> Result<Vec<Instruction>> {
         AccountMeta::new_readonly(accounts::ASSOCIATED_TOKEN_PROGRAM, false),
         fee_recipient_meta,
         AccountMeta::new(associated_quote_fee_recipient, false),
-        AccountMeta::new_readonly(buyback_fee_recipient, false),
+        AccountMeta::new(buyback_fee_recipient, false),
         AccountMeta::new(associated_quote_buyback_fee_recipient, false),
         AccountMeta::new(bonding_curve_addr, false),
         AccountMeta::new(associated_base_bonding_curve, false),
@@ -705,7 +705,7 @@ fn build_sell_v2(params: &SwapParams) -> Result<Vec<Instruction>> {
         AccountMeta::new_readonly(accounts::ASSOCIATED_TOKEN_PROGRAM, false),
         fee_recipient_meta,
         AccountMeta::new(associated_quote_fee_recipient, false),
-        AccountMeta::new_readonly(buyback_fee_recipient, false),
+        AccountMeta::new(buyback_fee_recipient, false),
         AccountMeta::new(associated_quote_buyback_fee_recipient, false),
         AccountMeta::new(bonding_curve_addr, false),
         AccountMeta::new(associated_base_bonding_curve, false),
@@ -937,6 +937,24 @@ mod tests {
         }
     }
 
+    #[test]
+    fn pumpfun_v2_buyback_fee_recipient_is_writable() {
+        let mint = pump_mint();
+        let mut params = swap_params_for_buy(mint, TOKEN_PROGRAM);
+        params.create_output_mint_ata = false;
+
+        let buy_ix = build_buy_v2(&params).unwrap().pop().unwrap();
+        assert!(global_constants::BUYBACK_FEE_RECIPIENTS.contains(&buy_ix.accounts[8].pubkey));
+        assert!(buy_ix.accounts[8].is_writable);
+
+        params.trade_type = crate::swqos::TradeType::Sell;
+        params.input_mint = mint;
+        params.output_mint = crate::constants::SOL_TOKEN_ACCOUNT;
+        let sell_ix = build_sell_v2(&params).unwrap().pop().unwrap();
+        assert!(global_constants::BUYBACK_FEE_RECIPIENTS.contains(&sell_ix.accounts[8].pubkey));
+        assert!(sell_ix.accounts[8].is_writable);
+    }
+
     #[test]
     fn pumpfun_v1_sell_fixed_output_uses_min_sol_directly() {
         let mint = pump_mint();