test: add GPU end-to-end CI coverage

[balasaajay]

What does this PR do?

Adds an end-to-end GPU test using the infrastructure introduced in #2630 and the test foundation from #2575.

The test exercises:

Qwen on GPU → vLLM → Gym model server → simple agent → string_match verifier

• Serves Qwen/Qwen2.5-0.5B-Instruct at a pinned Hugging Face revision.
• Runs locally inside the CI container without inference-provider API keys.
• Uses one GPU and temperature 0.
• Verifies a real rollout receives reward 1.0, uses the expected model, and contains valid token usage and output.
• Prefetches isolated Gym server environments during the container build.
• Persists the Hugging Face cache between runs.
• Uploads rollout, Gym, vLLM, model-readiness, and GPU diagnostic artifacts even when the test fails.
• Adds bounded readiness checks, evaluation timeouts, and graceful process cleanup.

Checklist

• I have read the contributing guidelines.
• The change is focused; unrelated "drive-by" edits are tracked as separate issues/PRs.
• Tests added or updated and pass locally, or N/A for docs-only / non-code changes (so CI unit/server checks pass when applicable).
• Pre-commit checks pass locally (pre-commit run --all-files) (so CI lint/format/copyright pass).
• All commits have DCO sign-off (git commit -s) (so the DCO check passes).