Add pending and available_including_pending fields to CreditBalance

Summary

This update extends the CreditBalance object with two new optional fields: pending and available_including_pending. These fields provide enhanced insight into card balances by including transactions that are authorized but not yet consolidated.

Updated endpoints

• No endpoint definitions changed.

Extensions changed

• CreditBalance schema now includes:
  • pending (integer, nullable): total amount in authorized-but-not-yet-consolidated Stripe transactions.
  • available_including_pending (integer, nullable): calculated as available - pending, clamped to zero, representing the real available spend headroom.

These fields are returned in responses from GET /cards and GET /cards/{id} under the balance object, allowing clients to better display the current card spend availability accounting for pending transactions.