feat(cli): add explicit span emitter

[rhewett-nv]

What does this PR do?

Adds the nemo-lens emit-spans CLI for exporting completed spans from explicit
epoch-second intervals in shell launch workflows.

Issues

N/A

Usage

nemo-lens emit-spans \
  --service nv.dl.launch \
  --span nv.dl.launch,1700000000.0,1700000003.0 \
  --span nv.dl.launch.container_load,1700000000.5,1700000001.5,nv.dl.launch

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

Validation on pr3-emit-spans-cli:

• ../.venv/bin/pre-commit run --all-files: passed
• PYTHONPATH=src ../.venv/bin/python -m pytest -q: 261 passed, 7 warnings
• ../.venv/bin/python -m compileall -q src tests: passed
• npm --prefix docs/fern run check: passed with 0 errors; one warning because the unauthenticated redirects check is skipped without fern login / FERN_TOKEN

Notes:

• PR3 is one signed commit: 0931973 feat(cli): add explicit span emitter.
• Invalid span input is validated before provider setup; if any span is invalid, nothing is emitted.
• --service sets service.name and takes precedence over service.name in OTEL_RESOURCE_ATTRIBUTES.
• New SDK-specific setup is confined to providers.py; nemo.lens.cli imports successfully with opentelemetry.sdk unavailable.
• No cross-repo fallback update is required because this adds a CLI entry point, not a new top-level Lens Python API symbol.

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[ahmadki]

/ok to test 866e71f

The merge-base changed after approval.