[codex] Split sandbox unit tests in CI

[hemildesai]

Summary

• add a sandbox pytest marker and apply it to the sandbox unit-test modules
• keep ng_dev_test for the core unit-test run while selecting not sandbox via PYTEST_ADDOPTS
• run sandbox unit tests in a separate pytest invocation
• append sandbox coverage into the same report so the overall coverage gate still runs once
• install the sandbox extra in the full-test-suite workflow so the marked sandbox tests have their optional dependencies
• make CLI did-you-mean parsing handle argparse's unquoted choose from a, b format, which was blocking the split core test command locally

Validation

• uv run pytest tests/unit_tests/test_cli_main.py::TestDidYouMean::test_mistyped_group tests/unit_tests/test_cli_main.py::TestDidYouMean::test_mistyped_action tests/unit_tests/test_cli_main.py::TestDidYouMean::test_mistyped_flag_choice -q
• PYTEST_ADDOPTS='-m "not sandbox" --cov-report= --cov-fail-under=0' uv run ng_dev_test
• uv run pytest --cov=. --cov-append --cov-report=term-missing --durations=10 -m sandbox
• uv run pre-commit run --files .github/workflows/unit-tests.yml .github/workflows/full-test-suite.yml pyproject.toml nemo_gym/cli/main.py tests/unit_tests/test_sandbox.py tests/unit_tests/test_opensandbox_provider.py tests/unit_tests/test_apptainer_provider.py

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