[SPARK-56952][CORE] Preserve heartbeat timeout executor loss reason

[sunchao]

What changes were proposed in this pull request?

This PR preserves the original loss reason when Spark replaces an executor after a heartbeat timeout.

Current flow:

1. HeartbeatReceiver detects that an executor has stopped heartbeating.
2. Spark creates ExecutorProcessLost("Executor heartbeat timed out ...").
3. Spark requests executor replacement.
4. The backend may later report the removal as generic ExecutorKilled.

Step 4 drops the more useful heartbeat-timeout diagnosis.

This PR keeps that timeout reason through the replacement flow, with two safeguards:

• use it only when the backend reports generic ExecutorKilled,
• do not override a more specific backend reason such as ExecutorExited.

It also clears the pending preserved reason if the kill request is rejected or fails.

Why are the changes needed?

Spark already knows that the executor was replaced because of a heartbeat timeout, but that information can be lost before the scheduler records the final executor loss reason.

Keeping the original reason makes executor-loss reporting more accurate and avoids collapsing a timeout-driven replacement into an ordinary generic kill.

This fixes SPARK-56952.

Does this PR introduce any user-facing change?

Yes.

Executor loss reporting is more specific for heartbeat-timeout removals. Cases that previously appeared as generic ExecutorKilled can now retain:

ExecutorProcessLost("Executor heartbeat timed out ...")

If the backend provides a concrete loss reason, Spark still keeps that backend reason instead.

How was this patch tested?

Unit tests cover:

• preserving the heartbeat-timeout reason when the backend reports ExecutorKilled,
• preserving a concrete backend-provided reason instead of overriding it,
• clearing the pending timeout reason when executor kill is rejected.

The OSS port itself has not been run through a local full test command yet.

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