Add insufficient funds/shares validation to ExecuteOrder service

[AbuMareBear]

Summary

The ExecuteOrder service is missing validation for:

• Canceling buy orders when user has insufficient funds
• Canceling sell orders when user has insufficient shares

What's Needed

1. Add validation logic to ExecuteOrder service to check:

   • For buy orders: user has enough cash balance to cover the purchase
   • For sell orders: user owns enough shares to sell

2. Cancel orders that fail validation instead of executing them

3. Add tests in test/services/execute_order_test.rb for:

   • Buy order with insufficient funds → order should be canceled
   • Sell order with insufficient shares → order should be canceled

[AbuMareBear]

@pollygee Not sure if this service needs to include this validation but I'm creating an issue incase it is. Feel free to close otherwise.