add pending balance and tests for confidetial transfer extension

[dvrvsimi]

Summary

This PR introduces a new helper for querying and decrypting the pending balance of a confidential token account, and adds comprehensive tests for confidential transfer account info logic.

Changes

• Adds get_pending_balance and related helpers to ApplyPendingBalanceAccountInfo.
• Adds robust tests for:
  • Pending balance decryption and combination
  • Available balance decryption and error handling
  • Total balance calculation and overflow handling
  • Utility function combine_balances (including overflow cases)
• Ensures all cryptographic edge cases and error paths are covered.

Motivation

• Makes it easier for client code to query pending balances in confidential transfer accounts, such cases.