chore: Refunder e2e tests

[squadgazzz]

Background

The ethflow refunder was broken for a long time, and the only way we could identify the issue was by looking into user reports. As a result, a hotfix was released: #3779.

Details

This logic is not covered by any e2e tests. While creating a more sophisticated alerting system should help to identify issues after the release, e2e tests should prevent us from merging a breaking change.

Acceptance criteria

e2e tests that cover the main logic of the refunder crate.

[Grinsven]

I can tackle this. Looking at the hotfix in PR #3779 to understand what broke, then I'll add e2e tests to prevent it happening again.

[Grinsven]

The existing test in crates/e2e/tests/e2e/refunder.rs covers the main refund flow and would have caught the #3779 bug (missing signer credential). It runs in CI via just test-e2e-local.

However, it doesn't test order filtering thresholds. The test uses 9999 bps slippage which will pass any threshold, so we're not actually validating that min_validity_duration and min_price_deviation work correctly.

Should add tests for:

• Orders below threshold (should NOT be refunded)
• Orders above threshold (should be refunded)
• Orders not yet valid long enough (should NOT be refunded)