feat: separate build and simulate steps in TransactionBuilder

[thinktanktom]

Description

Fixes #48

This PR separates the building and simulation steps in the TransactionBuilder API to provide more flexibility to users.

Changes

• New function: simulate_transaction(tx, env, account) - standalone function that simulates an already-built transaction
• Refactored: simulate_and_build() now internally calls build() then simulate_transaction()
• Improved efficiency: Removed unnecessary transaction reconstruction - now modifies transaction in-place using mut
• Backward compatible: Existing simulate_and_build() API continues to work exactly as before
• New test: Added test_simulate_transaction() to verify the new function works correctly
• Export: Added simulate_transaction to public API in lib.rs

New API Usage

Option 1: Build and simulate separately (new flexibility)

let tx = TransactionBuilder::new(&account, &env)
    .add_operation(operation)
    .build()
    .await?;

let simulated_tx = simulate_transaction(tx, &env, &account).await?;

Option 2: Build and simulate together (existing convenience)

let tx = TransactionBuilder::new(&account, &env)
    .add_operation(operation)
    .simulate_and_build(&env, &account)
    .await?;

Testing

• ✅ All existing tests pass
• ✅ New test test_simulate_transaction() added and passing
• ✅ cargo fmt applied
• ✅ cargo clippy clean

Checklist

• Code compiles without warnings
• All tests pass
• Code is formatted with cargo fmt
• Code passes cargo clippy
• Documentation includes examples
• Backward compatibility maintained