[codex] Make Lepton dependency optional

[hemildesai]

Summary

• Move leptonai out of the base dependency set and into a new lepton extra.
• Guard Lepton SDK and Ray helper imports so base nemo_run imports, scheduler mappings, and Ray wrapper modules do not require leptonai.
• Add tests that simulate leptonai being unavailable and skip Lepton-specific tests when the optional SDK is not installed.
• Update the docs install snippet for DGX Cloud Lepton to use the new extra.

Validation

• uv lock --check
• uv run --group lint -- ruff check nemo_run test/core/execution/test_lepton.py test/core/execution/test_lepton_optional_dependency.py test/run/ray/test_lepton.py
• uv run --group lint -- ruff format --check nemo_run test/core/execution/test_lepton.py test/core/execution/test_lepton_optional_dependency.py test/run/ray/test_lepton.py
• uv run -- pytest test/core/execution/test_lepton_optional_dependency.py
• uv run --extra lepton -- pytest test/core/execution/test_lepton.py test/run/ray/test_lepton.py

Note: the Lepton/Ray test command passed, then Ray printed an ignored ModuleNotFoundError: No module named 'ray.dag' from an atexit shutdown callback after pytest completed.