Fix heartbeat timer to prevent unnecessary pulls

[mtmk]

if application is hanging on the message in foreach loop (as we are yielding) we stop the timer until iteration moved on to stop timer kicking in and issuing extra pull requests. This doesn't affect message delivery but increments consumer sequence unnecessarily. this behavior is now consistent with the Go client.

fixes #793

[Copilot]

Pull Request Overview

This PR addresses the issue of unnecessary pull requests by pausing the heartbeat timer while yielding messages.

• Introduces calls to StopHeartbeatTimer and ResetHeartbeatTimer in asynchronous message iteration methods.
• Adds a heartbeat timer control method in NatsJSConsume and duplicates the logic in NatsJSFetch.

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
src/NATS.Client.JetStream/NatsJSConsumer.cs	Inserts heartbeat timer control calls to prevent unnecessary pull requests.
src/NATS.Client.JetStream/Internal/NatsJSConsume.cs	Adds StopHeartbeatTimer and modifies ResetHeartbeatTimer implementation.
src/NATS.Client.JetStream/Internal/NatsJSFetch.cs	Duplicates StopHeartbeatTimer with an idle timeout check to match heartbeat logic.

Comments suppressed due to low confidence (2)

src/NATS.Client.JetStream/Internal/NatsJSConsume.cs:168

• The StopHeartbeatTimer implementation in NatsJSConsume.cs does not check for an idle timeout (unlike NatsJSFetch.cs). Verify that this inconsistency in behavior is intentional to avoid unintended timer actions.

public void StopHeartbeatTimer() => _timer.Change(Timeout.Infinite, Timeout.Infinite);

src/NATS.Client.JetStream/Internal/NatsJSConsume.cs:170

• ResetHeartbeatTimer in NatsJSConsume.cs resets both the due time and period to _hbTimeout, whereas the pattern in NatsJSFetch.cs uses Timeout.Infinite for the period. Confirm that this difference is deliberate to ensure consistent timer behavior.

public void ResetHeartbeatTimer() => _timer.Change(_hbTimeout, _hbTimeout);

[scottf]

LGTM