fix(ri-exchange): a convertible RI with all-zero prices reads as free in reshape math #1188

Summary

A convertible RI whose FixedPrice, UsagePrice and RecurringHourlyAmount are all zero gets MonthlyCost 0. In cloud-commitments-go pkg/exchange/reshape.go, pricingGatePasses (:678) returns true for MonthlyCost <= 0, and costComponent (:220) scores it as neutral 0.5. So the RI reads as free in reshape recommendations, the same failure #1181 / #1187 fixed for a non-positive duration.

Evidence

• monthlyCostFromConvertibleRI (internal/api/handler_ri_exchange.go, internal/server/handler_ri_exchange.go) computes the cost as FixedPrice/term + (UsagePrice + RecurringHourlyAmount) * hours, with no check for an all-zero price set.
• A fully-upfront RI has FixedPrice > 0, so a real convertible RI with all three at zero is anomalous: it means missing pricing data, not a free RI.
• Related: an RI whose FixedPrice is missing while the hourly parts are populated is silently under-priced.

Proposed fix

Treat an all-zero price set like the non-positive duration in #1187: exclude the RI from cost-based reshape inputs with a warning. Alternatively, pass an explicit "price unknown" through the library so the pricing gate refuses rather than passes. Add tests for both shapes.

Found by review of #1187.