Add `batch_charge` fee support

[orunganiekan]

Complexity: High (200 points)

Description
batch::batch_charge() calls the internal charge logic but bypasses the fee deduction added in issue #1 because it duplicates charge logic rather than calling lib.rs::charge().

Requirements and context

• Refactor batch.rs to reuse the same fee-aware charge path
• Ensure ChargeResult variants still work correctly
• Add test: batch charge with fee configured deducts fee per user

Suggested execution

git checkout -b feat/batch-charge-fee

• Edit contract/src/batch.rs
• Add test to contract/src/test.rs

Acceptance criteria

• cargo test passes
• Fee applied consistently in batch and single charge

[Goodnessukaigwe]

@Goodnessukaigwe has applied to work on this issue as part of the Stellar Wave Program's 5th wave.

i will like to work on this issue

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

[drips-wave]

Congratulations, @Goodnessukaigwe! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on June 2, 2026.

🧑‍💻 @Goodnessukaigwe: 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

When opening a PR, please link it to this issue to ensure it gets tracked accurately. Points are awarded when this issue is marked as completed by the maintainer.

🤠 Repo maintainers: Please keep an eye on the contributor's progress and review their work before the due date. You can manage this issue, including adjusting its complexity and points, here.

🌊 Happy Wave 🌊