[CI] Use real vLLM ModelConfig object in init_device test

[hfan]

Description

The mock object breaks the test when the vllm code tries to use it.

Tests

Ran the failure test: pytest -v tests/worker/tpu_worker_jax_test.py::TestTPUWorker::test_init_device_with_provided_devices

CI: https://buildkite.com/tpu-commons/tpu-inference-ci/builds/4606#019a16b6-9bf8-4dc5-81d5-97d6bc35791a

Checklist

Before submitting this PR, please make sure:

• I have performed a self-review of my code.
• I have necessary comments in my code, particularly in hard-to-understand areas.
• I have made or will make corresponding changes to any relevant documentation.

[github-actions]

Description

Start with a short description of what the PR does and how this is a change from
the past.

The rest of the description includes relevant details and context, examples:

• why is this change being made,
• the problem being solved and any relevant context,
• why this is a good solution,
• some information about the specific implementation,
• shortcomings of the solution and possible future improvements.

If the change fixes a bug or a Github issue, please include a link, e.g.,:
FIXES: b/123456
FIXES: #123456

Tests

Please describe how you tested this change, and include any instructions and/or
commands to reproduce.

Checklist

Before submitting this PR, please make sure:

• I have performed a self-review of my code.
• I have necessary comments in my code, particularly in hard-to-understand areas.
• I have made or will make corresponding changes to any relevant documentation.