Summary

• Implement and verify the Soroban stream withdraw logic so recipients can claim time-accrued funds.
• Fix and extend tests around initialization, top-ups, withdraw behavior, and cancellation to match the intended streaming semantics.

Implementation details

• withdraw now relies on calculate_claimable to compute (now - last_update_time) * rate_per_second and caps at remaining deposited balance, marking the stream inactive when fully drained.
• Tests cover recipient-only access, inactive streams, time-based accrual over multiple withdrawals, and caps at remaining balance, plus cancelation flows that settle the recipient and refund the sender.

Issue

Closes #198.