Split message handling logic out of Executor

[dmjb]

Prior to this change, the interface of the Executor directly consumes messages from a message queue, and then publishes a message to another queue once the evaluation is complete. This leads to some issues with testing:

1. The tests must be asynchronous in order to simulate event
   publishing. If the tests fail while running make test, the test
   hangs until the timeout happens.
2. Errors which take place inside the Executor get swallowed into the
   message and telemetry code, meaning that it's hard to figure out why
   exactly a test has failed.

This PR splits the Executor into two: the ExecutorEventHandler which handles all interactions with message queues, and the Executor which contains the logic for entity evaluation minus event handling.

The intention is that the tests for ExecutorEventHandler can use a mock of the Executor. Since the message handling logic is unlikely to change often, this should reduce the situations where the async tests fail and we end up with long hangs in the test. This test cleanup work will be implemented in a separate PR.

Summary

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Fixes #(related issue)

Change Type

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• Bug fix (resolves an issue without affecting existing features)
• Feature (adds new functionality without breaking changes)
• Breaking change (may impact existing functionalities or require documentation updates)
• Documentation (updates or additions to documentation)
• Refactoring or test improvements (no bug fixes or new functionality)

Testing

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Review Checklist:

• Reviewed my own code for quality and clarity.
• Added comments to complex or tricky code sections.
• Updated any affected documentation.
• Included tests that validate the fix or feature.
• Checked that related changes are merged.

[coveralls]

coverage: 52.484% (-0.003%) from 52.487%
when pulling 6e068ab on split-engine-consumer
into cdb5ae7 on main.

[coveralls]

coverage: 52.479% (-0.008%) from 52.487%
when pulling 6e068ab on split-engine-consumer
into cdb5ae7 on main.

[coveralls]

coverage: 52.443% (+0.01%) from 52.432%
when pulling 37c3d99 on split-engine-consumer
into d045d12 on main.

[blkt]

This looks great, thanks for working on this!