Let activities heartbeat during worker shutdown

[baekgyu-kim]

What was changed

• On the worker-shutdown path (heartbeatExecutor already shut down), HeartbeatContextImpl.heartbeat() now emits the heartbeat to the server before throwing ActivityWorkerShutdownException (previously: thrown without sending).
• Throttled like the normal scheduled path (success → heartbeat interval, transient failure → short retry interval), reusing the last heartbeat's throttle state so a caller looping on heartbeat() cannot flood the server.
• Send errors are swallowed (WARN-logged); ActivityWorkerShutdownException is always thrown, so a transient failure cannot mask the shutdown signal.
• ActivityWorkerShutdownException Javadoc updated accordingly.

Why?

• Before: on shutdown, heartbeat() threw immediately without contacting the server → no heartbeat during the awaitTermination grace period → server times the activity out and retries it → duplicate executions, despite the worker deliberately giving the activity time to finish.
• After: the activity can keep heartbeating during the grace period (by catching the exception) → each call refreshes the server-side heartbeat deadline → no premature timeout/retry.
• Design: emit-then-throw is the approach agreed in Add the ability to keep heartbeating while the worker is shutting down #2075; throttling keeps a manual heartbeat loop from flooding the server.

Checklist

1. Closes Add the ability to keep heartbeating while the worker is shutting down #2075

2. How was this tested:

   New HeartbeatContextImplTest cases:

   • emit-then-throw on the shutdown path
   • exception still thrown when the send fails (gRPC and non-gRPC)
   • repeated rapid calls throttled to a single send
   • prompt retry after a failed send (short interval, not a full one)
   • resend once the throttle interval elapses
   • throttle carry-over from a preceding normal heartbeat — fails if the throttle-state write is removed

   ./gradlew :temporal-sdk:test --tests "io.temporal.internal.activity.HeartbeatContextImplTest"
   ./gradlew :temporal-sdk:spotlessJavaCheck
   

3. Any docs updates needed?

   • The behavior is documented in the ActivityWorkerShutdownException Javadoc.