fix: portfolio websocket EventType parsing error

[feeeei]

This is a known bug in Go's encoding/json standard library: the package ignores case sensitivity when parsing JSON struct tags.

Minimal reproduction example:

package main
import (
	"encoding/json"
	"fmt"
)
func main() {
	var event struct {
		EventType string `json:"e"`
	}
	// JSON contains same field with different cases
	message := []byte(`{"e":"listenKeyExpired","E":1750063270815}`)
	if err := json.Unmarshal(message, &event); err != nil {
		fmt.Println(err) // outout: json: cannot unmarshal number into Go struct field .e of type string
	}
	fmt.Println(event)
}

Related Go official issue:
golang/go#14750

The simplest current workaround is to ensure struct member names exactly match the JSON field case.

[carlosmiei]

@feeeei Thanks a lot for fixing it, we will review and merge it shortly.

[feeeei]

@carlosmiei

The current Portfolio WebSocket code appears to be copied from the Futures module without thorough real-world testing.

Specific issues include:

1. The actual WebSocket returns a numeric timestamp (e.g., "E": 1750142342023), but tests artificially changed it to a string type to avoid reproducing the bug;
2. In USER-DATA streams:

• The position field in ACCOUNT_UPDATE events contained incorrect data;
• ORDER_TRADE_UPDATE events lacked critical fields like wt and ot.

API DOCS LINK:

• ACCOUNT_UPDATE
• ORDER_TRADE_UPDATE

All mentioned issues have been fixed.

[carlosmiei]

@feeeei Thanks once again