Implement stream flow rate modification after creation

[Chuks-coderr]

Overview

Add a modify_flow_rate instruction that allows the sender to change the flow rate on an active stream, settling accumulated balance at the old rate first.

Acceptance Criteria

• Current claimable balance locked at the modification timestamp
• New flow rate applies from modification timestamp onward
• end_time recalculated based on remaining balance and new rate
• Only sender can modify rate
• Tests: modify rate up and down; verify claimable is continuous

[NeduTheDev1]

@NeduTheDev1 has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Please assign to me maintainer i can get it done no issues

ℹ️ Repo Maintainers: To accept this application, review their application or assign @NeduTheDev1 to this issue.

[drips-wave]

Congratulations, @ezechinonyelumvivian-create! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on June 30, 2026.

🧑‍💻 @ezechinonyelumvivian-create: Please resolve the issue such that the repo's maintainers have enough time to review your contribution before the due date. You'll earn Points for completing the issue on-time, which will make you eligible for a share of the Stellar Wave Program's reward pool.

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