BatchWallet: convert duplicate-within-batch panic into per-item `Failure`

[snowrugar-beep]

• Problem Statement. contracts/batch-wallet/src/lib.rs
batch_create_wallets panics (DuplicateWallet = 7) when the
  same WalletCreateRequest appears twice in one batch. A
  partial-success outcome is more composable.
• Why it matters. Atomic-or-nothing is restrictive against
  rescue/restoration flows that prefer best-effort semantics.
• Expected Outcome. When duplicates appear in a batch, return
  them as WalletCreateResult::Failure(owner, code) while processing
  the others. Tests prove partial-success behavior.
• Acceptance Criteria.
  1. New test: send N+1 requests, M duplicates → result reports
     failed=M, others succeed, event wallet_duplicate emitted per
     duplicate.
  2. Document the new contract in README.
• Files / Modules. contracts/batch-wallet/src/lib.rs.
• Difficulty. Medium.
• Effort. 1 day.
• Labels. contracts, batch-wallet, needs-spec.

[gbengaeben]

Hello Maintainer

can i be assigned this issue to fix
i have thoroughly gon through it and i understand the modalities and neccessary fix

[grantfox-oss]

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Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #129)
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[grantfox-oss]

🎉 This issue has been marked as completed on GrantFox as part of the Official Campaign | FWC26 campaign!

@gbengaeben's PR #397 was approved and merged by @snowrugar-beep.

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👏 Great work, @gbengaeben! Keep contributing to VertexChainLabs.