feat(payment): add batch payment proof verification

[johnsaviour56-ship-it]

Summary

This PR introduces batch payment proof verification, enabling multiple payment proofs to be verified in a single contract call while preserving independent verification results for each proof.

Changes

• Added verify_payment_proofs_batch() to process multiple payment proofs in one request.
• Reused the existing single-proof verification logic for each proof in the batch.
• Preserved input ordering by returning results in the same order as the submitted proofs.
• Enforced a maximum batch size of 20 proofs per call to prevent excessive resource consumption.
• Ensured invalid or stale proofs do not interrupt verification of other proofs in the batch.
• Added comprehensive tests covering mixed verification scenarios and batch size validation.

Testing

Added coverage for:

• Successful verification of multiple valid proofs
• Mixed batches containing valid and stale/invalid proofs
• Preservation of result ordering
• Batch size limit enforcement
• Regression testing for existing payment proof verification

Acceptance Criteria

• Returns a result vector matching the input order and length
• Rejects batches containing more than 20 proofs
• Invalid or stale proofs do not prevent verification of other entries
• Added tests for mixed valid/invalid batches
• All existing tests pass
• cargo clippy passes with zero warnings

Result

Payment proof verification is now more efficient for off-chain reconciliation workflows by supporting batch processing while maintaining deterministic ordering, isolated per-proof results, and bounded resource usage.

Closes #227

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