feat: Add optional heartbeat response handling

[Copilot]

Adds opt-in heartbeat response monitoring. By default, heartbeats are sent automatically but responses are not delivered to the subscription channel, avoiding noise for applications that don't need connection health monitoring.

Changes

• API: Added return_heartbeat_response(bool) method to all plant handles for runtime control
• Default behavior: Heartbeat responses ignored unless explicitly enabled (ignore_heartbeat_response: true by default)
• Command routing: Added SetHeartbeatResponseMode command and updated SendHeartbeat to accept ignore_response parameter
• Response handling: Modified request_handler to respect heartbeat response mode when callbacks are registered

Usage

// Enable heartbeat monitoring during trading hours
handle.return_heartbeat_response(true).await;

// Disable during off-market hours to avoid false alarms
handle.return_heartbeat_response(false).await;

Breaking Change

Partially reverts 0.5.0 behavior where all heartbeat responses were automatically delivered. Applications relying on heartbeat responses in the subscription channel must now call return_heartbeat_response(true).

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