[Tests] Add tests for headless internal DP LB

[njhill]

This was a gap in our DP tests.

Includes moving some common prometheus-based load-balancing verification to test_utils.py.

[gemini-code-assist]

Code Review

This pull request adds important tests for the headless internal data-parallel load balancer, which was a notable gap. The refactoring of Prometheus-based verification logic into test_utils.py is a great move for code reuse and maintainability.

The new tests in test_internal_lb_dp.py are comprehensive. My primary feedback is to replace the time.sleep() calls used for server synchronization with a more robust polling mechanism. This will prevent potential flakiness in the CI pipeline, which is critical for test stability.

[github-actions]

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