chore: avoid constant local run UUIDs

[rhewett-nv]

What does this PR do?

Generates missing run IDs once and uses them to derive distinct deterministic local run UUIDs.

Issues

N/A

Usage

from nemo.lens import NemoLensConfig, ensure_run_id

config = NemoLensConfig.from_env()
run_id = ensure_run_id(config)

# Before your PR is "Ready for review"

Pre checks:

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

# Additional information

- The optional-dependency fallback surface is unchanged.

[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 45aa84d

The merge-base changed after approval.

[ahmadki]

/ok to test 408b39f

The merge-base changed after approval.