Create commitments

Creates one or more usage commitments on a contract, synchronously, and returns their IDs. Each commitment links one or more usage-based billing terms and, optionally, configures prepaid, overage, and true-up billing for the commitment period. Overage pricing (`overageConfigs`) can be a single flat rate or multiple tiers via `overagePricings`; when tiered, `tierPricingType` selects how the tiers are applied — USE_ALL_TIER_PRICES (graduated, the default) charges each tier's rate for the portion of usage within it, while USE_TOP_TIER_PRICE_ONLY (volume) charges the single rate of the tier the total usage falls into. Fails with a validation error if a billing term is locked down (already invoiced) or otherwise ineligible for commitment linkage, or if the contract does not exist for this manufacturer.