tests(pool): no test for funding the same invoice twice

[K1NGD4VID]

Summary

The pool's fund_invoice checks both the invoice status and its own FundedInvoice key. There is no test verifying double-funding is prevented through both checks.

Current Behavior

fund_invoice at contracts/pool/src/lib.rs:351-467 checks invoice status and FundedInvoice key. If someone tries to fund an already-funded invoice, the pool's own check catches it. But there is no test for this scenario.

Expected Behavior

Add tests for calling fund_invoice twice with the same invoice, and for calling fund_invoice after repayment/default.

Acceptance Criteria

• Test: double fund_invoice panics with AlreadyFunded
• Test: fund_invoice after repayment (should fail)
• Test: fund_invoice after default (should fail)

Tech Stack

contracts/pool/src/test.rs