test: verify import isolation from Otel SDK

[shangrex]

What does this PR do?

Adds a subprocess test verifying that nemo.lens can be imported without requiring opentelemetry-sdk, preserving optional SDK dependency isolation.

Issues

None.

Usage

No user-facing usage changes. Run the focused test with:

uv run pytest -q test_import_isolation.py

Before your PR is "Ready for review"

Pre checks:

• I read the Contributor guidelines.
• All commits are signed off (git commit -s) per the DCO.
• I added or updated tests under tests/ for any behavior change.
• I ran pytest and pre-commit run --all-files locally; both pass.
• I updated docs/ and README.md where behavior, configuration, or public API changed.
• If this changes a lens public API symbol, I updated the cross-repo _fallbacks.py files in Megatron-LM / RL / Gym.

Additional information

[copy-pr-bot]

This pull request requires additional validation before any workflows can run on NVIDIA's runners.

Pull request vetters can view their responsibilities here.

Contributors can view more details about this message here.

[ahmadki]

/ok to test 6f3c2bc

[ahmadki]

/ok to test 0e61533

[ahmadki]

/ok to test 675e4c3