testclient: add regression test for LoadSimulationController executor…

[ChimdumebiNebolisa]

What does this PR do?

Fixes an executor thread leak in LoadSimulationController and adds/updates tests to prevent regressions.

Why is this needed?

LoadSimulationController previously used a cached ExecutorService without a shutdown path, which can leave non-daemon threads running and delay or prevent clean JVM shutdown in embedded or containerized test environments.

What changed?

• Converted the cached executor from static to instance-scoped.
• Implemented AutoCloseable and added a close() method that shuts down the executor safely.
• Ensured cleanup via finally in run().
• Updated the JUnit 5 test to assert no leaked non-daemon pool threads remain after close().

How was this tested?

Updated JUnit 5 test creates a controller instance, forces executor thread creation, calls close(), and verifies the executor threads terminate.

Docs

• doc
• doc-required
• doc-not-needed
• doc-complete

[github-actions]

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