Add onTerminate hook to handle cluster termination

[Copilot]

Overview

This PR adds an optional onTerminate callback to the cluster that is invoked when the cluster terminates, addressing the need to handle cluster termination events.

Motivation

When a cluster terminates (either explicitly via end() or automatically due to failures), there was no way for applications to be notified and perform cleanup operations or logging. This hook provides a clean mechanism to handle these scenarios.

Changes

Core Implementation

• Added optional onTerminate parameter to createCluster options
• Modified terminateCluster function to invoke the hook after all nodes are killed
• Added proper error handling for fire-and-forget termination calls to prevent unhandled promise rejections

Usage

const cluster = await createCluster({
  numNodes: 2,
  onTerminate(reason) {
    console.log(`Cluster terminated: ${reason}`)
    // Perform cleanup operations, logging, etc.
  }
})

The hook receives a reason parameter indicating why the cluster was terminated:

• 'cluster stopped' - when cluster.end() is called explicitly
• 'Too many consecutive failures' - when automatic termination occurs due to node startup failures

Testing

Added comprehensive test coverage:

• Test for explicit termination via end() method
• Test for automatic termination on too many consecutive failures
• Both tests verify the hook is called with the correct reason

All existing tests continue to pass, and code coverage is maintained at 91.21%.

Backwards Compatibility

This is a non-breaking change. The onTerminate hook is optional and existing code will continue to work without modification.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Add onTerminate hook to handle cluster termination</issue_title>
<issue_description>When a cluster terminates, we have no way to handle this.
We should have an optional onTerminate.</issue_description>

<agent_instructions>Please update unit tests and coverage.</agent_instructions>

Comments on the Issue (you are @copilot in this section)

Fixes #7

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