[ROCm][CI] Fix engine teardown and text normalization to stabilize voxtral test

[AndreasKaratzas]

Fixes two independent failures in test_voxtral_realtime_forward and test_voxtral_realtime_generator observed on ROCm CI runs.

ROCm non-determinism (test_voxtral_realtime_forward)

• Adds ROCM_ENGINE_KWARGS to tests/utils.py as the Python-API equivalent of the existing ROCM_EXTRA_ARGS (CLI flags).
• Applies max_num_seqs=1 and enable_prefix_caching=False on ROCm via **ROCM_ENGINE_KWARGS in ENGINE_CONFIG, eliminating batch variance and prefix-cache interference that caused non-deterministic outputs even at temperature=0.0.

Missing text normalization (test_voxtral_realtime_forward)

• The model occasionally transcribes "OBS" as "a base hit" and "oh, my" as "oh my", which are acoustically valid alternatives.
• test_voxtral_realtime_generator already handled this inline. Extracted into a shared _normalize() helper and applied to both tests to eliminate the drift.

Engine teardown / OOM (test_voxtral_realtime_generator)

• The engine fixture had no teardown, so the LLM held GPU memory when the async engine's subprocess tried to start, causing Engine core initialization failed with Failed core proc(s): {}.
• Fixed by converting the fixture to a yield fixture that calls llm.llm_engine.shutdown() and torch.cuda.empty_cache() after the test.

Assertion clarity (both tests)

• Replaced single-list assert texts == EXPECTED_TEXT with per-element assertions that print got / expected on failure, removing the need for -vv to see the actual diff.

cc @kenroche

[gemini-code-assist]

Code Review

This pull request introduces several important fixes to stabilize the voxtral tests on ROCm. The changes include adding ROCm-specific engine arguments to ensure deterministic outputs, refactoring text normalization into a shared helper function, and improving assertion messages for better debugging. A key part of this PR is the fix for an engine teardown issue that caused OOM errors, which is addressed by converting fixtures to yield fixtures with cleanup logic.

My review focuses on the correctness of this new teardown logic. I've identified a critical issue in the shutdown sequence for the synchronous engine fixture that will likely prevent the resource leak from being fixed. Please see my detailed comment.