Add missing identity to Heartbeat RPC

[yuandrew]

What was changed

• Send the initial worker heartbeat immediately instead of waiting one full heartbeat
  interval.
• Leave elapsed_since_last_heartbeat unset on the first heartbeat.
• Add test confirming RecordWorkerHeartbeatRequest.identity is sent.

Why?

Match behavior with Java and Rust

Checklist

1. Closes

2. How was this tested:

3. Any docs updates needed?

---

Note

Low Risk
Observability and RPC field population only; no task execution or auth changes, with new unit tests covering the behavior.

Overview
Aligns Go worker heartbeat behavior with the Java and Rust SDKs.

The shared namespace heartbeat loop now fires one RecordWorkerHeartbeat as soon as it starts, instead of waiting a full interval. Worker callbacks are registered before that goroutine starts so the first batch includes the new worker.

RecordWorkerHeartbeatRequest now sets Identity from the Temporal client (ClientOptions.Identity). Each WorkerHeartbeat leaves elapsed_since_last_heartbeat unset on the first report and only populates it after a prior heartbeat timestamp exists.

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