Require metrics for metric-scoped list commands

[fanruan-tuco]

Summary

• mark --metrics as required for mf list dimensions and mf list entities
• replace the misleading all-object command descriptions with metric-scoped wording
• add shared regression coverage for help output and missing-option errors
• add a changelog entry for the CLI fix

Both commands already require at least one metric through SequenceParamType(min_length=1). Replacing the empty-string defaults with required=True makes the Click contract match the existing behavior and produces the standard missing-option error.

Testing

• hatch run dev-env:pytest tests_dbt_metricflow/cli/test_cli.py (16 passed)
• dbt-metricflow pre-commit: ruff, black, mypy, and file checks passed
• changie batch --dry-run 999.999.999 passed

Fixes #2109

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[fanruan-tuco]

CLA submitted.