honor the operator on credit balance conditions

[ryanechternacht]

When credits run out on a credit burndown entitlement, CheckFlag falls through to the flag default with rule_type: null and "No rules matched". The API writes two rules for the entitlement, an entitling plan_entitlement rule whose credit condition uses lt and a plan_entitlement_usage_exceeded rule whose condition uses gte, but checkCreditBalanceCondition ignored the operator and evaluated balance >= cost for both. The exceeded rule could never match, so a drained balance was indistinguishable from a plan that lacks the feature, and a flag with a true default kept entitling companies with zero credits.

The operator now picks the direction of the balance check: gt/gte match when the balance does not cover the cost, everything else when it does. Overage still short-circuits to "covered", so the exceeded rule stays quiet when overage is on. A drained balance returns value: false with rule_type: plan_entitlement_usage_exceeded, matching numeric entitlements. The API only writes credit conditions through NumericLimitCondition, which has always used lt for the entitling rule and gte for the exceeded rule, so existing rules pick up the fix without a migration.

Mirrors the same change to rulesengine-rust in schematichq/schematic-api. The API needs a go.mod bump to the tag cut from this.