tests(pool): no test for handle_default on unknown invoice

[K1NGD4VID]

Summary

The pool's handle_default function at contracts/pool/src/lib.rs:687-744 returns false when no FundedInvoice entry exists. This early-return path has no test coverage.

Current Behavior

Lines 696-698:

if !env.storage().persistent().has(&funded_key) {
    return false;
}

The test suite has no test that calls handle_default with an unknown invoice ID.

Expected Behavior

Add a test that calls pool.handle_default with an invoice ID that was never funded.

Acceptance Criteria

• Test: handle_default returns false for unfunded invoice
• Test: pool state unchanged

Tech Stack

contracts/pool/src/test.rs