fix: Add support for multi-token batch routing in a single transaction

[Chummy-debug]

Summary

Implemented support for multi-token batch routing in a single transaction for issue #513.

The Payment struct already explicitly included a token_address field per payment, and route_payments already iterated each payment and dispatched to process_single_payment using each payment's own token address — so the core routing logic already handled mixed tokens correctly.

The missing piece was test coverage. I added test_route_payments_multi_token_batch in payment_router/src/lib.rs which:

• Creates two distinct tokens (USDC-like and EURC-like)
• Routes a batch of two payments with different tokens and different recipients in a single route_payments call
• Verifies each token's sender balance, recipient balance, and treasury fee are correct (100 bps fee: 20 for the 2000 USDC payment, 10 for the 1000 EURC payment)
• Verifies the sender's cumulative volume aggregates across both tokens (3000)

All three acceptance criteria are satisfied: (1) Payment supports individual token addresses, (2) batch routing handles mixed tokens, (3) tests verify multi-token routing succeeds.

Changed files

• payment_router/src/lib.rs

Test plan

Run the contract test suite:

• cargo test -p payment_router (or cargo test in the payment_router directory)

This will run the new test_route_payments_multi_token_batch test along with all existing tests. The test verifies multi-token batch routing succeeds with correct per-token fee deductions, recipient credits, and aggregated sender volume.

This PR was created as a draft by the issue solver bot. It will remain a
draft until repository CI passes.

Closes #513

[vercel]

@issue-solver-bot is attempting to deploy a commit to the Abdulazeem's projects Team on Vercel.

A member of the Team first needs to authorize it.

[drips-wave]

@Chummy-debug Great news! 🎉 Based on an automated assessment of this PR, the linked Wave issue(s) no longer count against your application limits.

You can now already apply to more issues while waiting for a review of this PR. Keep up the great work! 🚀

Learn more about application limits

[Abdulazeem-code]

KINDLY RESOLVE CONFLICTS