Add Batch Subscription Functionality

[Abd-Standard]

Description
Allow users to subscribe to multiple notification channels within a single transaction to improve efficiency.

Acceptance Criteria
Multiple subscriptions are processed successfully.
Failed subscriptions do not corrupt state.
Gas usage is documented.

[GazzyLee]

@GazzyLee has applied to work on this issue as part of the Stellar Wave Program's 7th wave.

I'd like to take this on. I'll implement support for processing multiple notification channel subscriptions atomically within a single transaction, ensure failures are handled safely so state remains consistent, and document the resulting gas usage along with test coverage for successful and failure scenarios. ETD is in 5 hours tops, please assign

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

[king-aj-the-first]

@king-aj-the-first has applied to work on this issue as part of the Stellar Wave Program's 7th wave.

Helloo Repo Maintainer, I'd like to work on this issue

ℹ️ Repo Maintainers: To accept this application, review their application or assign @king-aj-the-first to this issue.

[drips-wave]

Congratulations, @king-aj-the-first! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on July 30, 2026.

🧑‍💻 @king-aj-the-first: 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.

Warning

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