Add max cost cap to Predict mint

[emmazzz]

Summary

• Require max_cost on the existing Predict mint entrypoint and internal mint path.
• Abort with EMintCostAboveMax when the all-in mint withdrawal exceeds the cap.
• Add focused flow coverage for exact-cap success and cap-underflow abort.

Key decisions

• Keep the cap non-optional; callers can pass std::u64::max_value!() for uncapped behavior.
• Split mint preparation/payment helpers so fixed-cost mint can stack separately without duplicating flow gates.

Test plan

• sui move test --path packages/predict mint_slippage --gas-limit 100000000000
• sui move build --path packages/predict --warnings-are-errors
• sui move test --path packages/predict --gas-limit 100000000000