Fix order submission for single leg orders

[orkhanM]

Was getting strange errors and took a while to debug (error handling should probably expose the api response for easier debugability) some of the issues:

• it seems like POST /accounts/{accountNumber}/orders returns a 201 response code to indicate success. (see doc)
• the order quantity in the api request body has to be an integer (see docs).
• the trailing comma in the LegTemplate was causing issues

I've changed:

• the order submission to use sonic.Marshal (as it's used in other places in this codebase) instead of fmt.Sprintf .
• the Quantity for SingleLegOrder to be an int to conform with the api spec
• the struct to marshal correctly. I noted there are also other structs that don't seem to be fully implemented yet (currently in the works?) so I didn't modify them however if they're in-the-works then it'd make sense to have them follow the same pattern.
• added 201 as an acceptable response code to agent.Handler

Here is the order submission request body before and after these updates:

Before:

{
  "orderType": "MARKET",
  "session": "NORMAL",
  "duration": "DAY",
  "orderStrategyType": "SINGLE",
  "orderLegCollection": [
    
{
  "instruction": "BUY",
  "quantity": 1.000000,
  "instrument": {
    "symbol": "AAPL",
    "assetType": "EQUITY"
  }
},

  ]
}

{"time":"2025-02-20T20:21:29.210308705-05:00","level":"ERROR","msg":"Failed to submit order","!BADKEY":"server is freaking out: server is freaking out"}

after:

{"orderType":"MARKET","session":"NORMAL","duration":"DAY","orderStrategyType":"SINGLE","orderLegCollection":[{"instruction":"BUY","quantity":1,"instrument":{"symbol":"GOOG","assetType":"EQUITY"}}]}
 
201

🍻

[samjtro]

beauty! sorry it's been an insane last few weeks, got really sick and i've been swamped at work. will get this & the other pr merged in tonight.

really appreciate your great work here!