[FEAT] Soroban: Implement cancel_stream Logic

[ogazboiz]

Description

Allow senders to proactively close a stream and recover unused funds.

Tasks

• Verify the caller matches the stream sender.
• Calculate and payout the final claimable amount to the recipient.
• Refund all remaining deposited_amount back to the sender.
• Mark the stream as is_active = false.

Context

Essential for user trust and flexibility in managing their payment commitments.

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[CMI-James]

@CMI-James has applied to work on this issue as part of the Stellar Wave Program's 2nd wave.

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Congratulations, @CMI-James! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on February 26, 2026.

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