Improve /run/update/ detection by replacing string matching with JSON parsing

[pratham15541]

Reference: https://github.com/omegaup/quark/blob/main/cmd/omegaup-broadcaster/main.go#L293-L296

📝 Issue Description

Currently, the detection of /run/update/ messages relies on string matching:

strings.Contains(message.Message, "\"message\":\"/run/update/\"")

Suggested Improvement

Parse the JSON payload and check the message field explicitly:

type payload struct {
    Message string `json:"message"`
}

var p payload
if err := json.Unmarshal([]byte(message.Message), &p); err == nil {
    if len(message.Contest) > 0 && strings.HasPrefix(p.Message, "/run/update/") {
        contestChan <- message.Contest
    }
}