feat: add multi-token pool support integration test

[sanmipaul]

Summary

`contracts/tests/integration_tests.rs` has no test covering multi-token pool operations. The pool supports multiple tokens (USDC, XLM, etc.) but all existing integration tests use a single token. Edge cases unique to multi-token pools are completely untested.

Missing Test Cases

1. Deposit USDC and XLM in the same pool, verify balances are tracked separately
2. Fund one invoice in USDC and another in XLM, verify each uses the correct token
3. Repay USDC invoice with USDC (correct) vs. XLM (should fail)
4. Yield accrual is calculated per-token, not pooled across tokens
5. Withdraw USDC does not affect XLM balance and vice versa
6. Remove one token while other remains active — verify isolation

Implementation Notes

• Use Soroban test environment's `register_stellar_asset_contract()` to create multiple mock tokens
• The test should cover at least two tokens simultaneously
• Use the existing `integration_tests.rs` helper functions for setup

Acceptance Criteria

• Test `test_multi_token_deposit_isolation` created and passing
• Test `test_multi_token_fund_and_repay` created and passing
• Test `test_cross_token_repay_rejected` created and passing
• All new tests run in CI without additional configuration

References

• `contracts/tests/integration_tests.rs`
• `contracts/pool/src/lib.rs` — multi-token support

[kimanicode]

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

I would like to work on this issue because untested multi-token edge cases in a DeFi pool are a real risk and I have experience writing Soroban integration tests. I will implement all three required tests covering deposit isolation, cross-token funding and repayment, and rejected cross-token repay, using the existing helper functions and mock token setup. All tests will pass in CI without additional configuration.

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

[mallam458]

@mallam458 has applied to work on this issue as part of the Stellar Wave Program's 5th wave.

i will love to work on this issue

ℹ️ Repo Maintainers: To accept this application, review their application or assign @mallam458 to this issue.

[drips-wave]

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

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