LITE-33042 Support monthly_trial billing period

[pcaro]

Summary

• Add monthly_trial to the BILLING_PERIOD tuple in constants.py
• Add monthly_trial: ('-',) to ALLOWED_COMMITMENTS (trial periods allow no long-term commitment)
• Fix _get_billing_period() to pass monthly_trial through as-is (prevents crash on space-split for multi-year conversion)

Test plan

• poetry run pytest tests/plugins/product/sync/test_items.py -x -q — all pass
• Updated test_validate_wrong_period_reservation expected error message to include monthly_trial
• New test test_validate_monthly_trial_period_wrong_commitment covers commitment restriction
• Manually clone a product with monthly_trial items and confirm 0 errors

Closes LITE-33042

🤖 Generated with Claude Code

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[sonarqubecloud]

Quality Gate failed

Failed conditions
66.7% Coverage on New Code (required ≥ 95%)

See analysis details on SonarQube Cloud