Add throttled high-rate MQTT publish warnings in runtime client

[Copilot]

High outbound MQTT publish rates could occur silently, making accidental publish loops hard to detect during runtime. This change adds lightweight rate monitoring in the publish path and emits console warnings only when publish throughput is unusually high.

• Publish-rate detection at MQTT egress

  • Added rate tracking in ArenaMqttClient.Publish(...), where scene/object/camera outbound messages converge.
  • Computes observed throughput over a short rolling window and compares against a warning threshold.

• Operator-facing warning behavior

  • Emits Debug.LogWarning when publish rate exceeds the threshold.
  • Warning text includes observed rate and threshold, plus the relevant update knobs (globalUpdateMs, objectUpdateMs, cameraUpdateMs).

• Noise and safety controls

  • Added warning cooldown to prevent log spam under sustained high throughput.
  • Synchronized rate-tracking state to keep counters/timestamps consistent if publish calls arrive concurrently.

float msgsPerSecond = publishedMsgsInWindow / elapsed;
if (msgsPerSecond >= publishRateWarningThresholdMsgsPerSecond &&
    now - lastPublishRateWarningTime >= publishRateWarningCooldownSeconds)
{
    Debug.LogWarning(
        $"High MQTT publish rate detected: {msgsPerSecond:F1} messages/s over {elapsed:F1}s " +
        $"(threshold: {publishRateWarningThresholdMsgsPerSecond:F0} messages/s). " +
        "Consider reducing update frequency (globalUpdateMs, objectUpdateMs, cameraUpdateMs)."
    );
}