[Solana]: Support adding instruction at specified index (#4415)

* [Solana]: Support adding instruction at specified index

* Rename tw_solana_transaction_add_instruction to tw_solana_transaction_insert_instruction

---------

Co-authored-by: Sergei Boiko <127754187+satoshiotomakan@users.noreply.github.com>

Author: 10gic

rust/chains/tw_solana/src/modules/utils.rs

@@ -175,15 +175,30 @@ impl SolanaTransaction {
             .tw_err(SigningErrorType::Error_internal)
     }
 
-    pub fn add_instruction(encoded_tx: &str, instruction: &str) -> SigningResult<String> {
+    pub fn insert_instruction(
+        encoded_tx: &str,
+        insert_at: i32,
+        instruction: &str,
+    ) -> SigningResult<String> {
         let tx_bytes = base64::decode(encoded_tx, STANDARD)?;
         let mut tx: VersionedTransaction =
             bincode::deserialize(&tx_bytes).map_err(|_| SigningErrorType::Error_input_parse)?;
 
         let instruction: Instruction =
             serde_json::from_str(instruction).map_err(|_| SigningErrorType::Error_input_parse)?;
 
-        tx.message.push_instruction(
+        if insert_at >= 0 && insert_at as usize > tx.message.instructions().len() {
+            return Err(SigningError::from(SigningErrorType::Error_invalid_params));
+        }
+
+        let final_insert_at = if insert_at < 0 {
+            tx.message.instructions().len() // Append to the end if negative
+        } else {
+            insert_at as usize // Use the specified position
+        };
+
+        tx.message.insert_instruction(
+            final_insert_at,
             instruction.program_id,
             instruction.accounts,
             instruction.data,