fix #4974: A thread leak issue in scaffolding unittest

[ccs96307]

PR title

[#4974][fix] Fix thread leak in scaffolding unittest by explicitly shutting down ScaffoldingLlm

Description

There is a thread leak in tests/unittest/scaffolding/test_bench.py caused by a missing shutdown() call on ScaffoldingLlm-based workers.

This issue causes the threadleak hook to fail during test execution.

This PR adds an explicit .shutdown() to ensure proper resource cleanup and allow the unit test to pass.

Test Coverage

• tests/unittest/scaffolding/test_bench.py
• Verified that pytest now passes

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