Fix broken swap-core unit tests

[dicnunz]

Summary

• Forward the test-only static-fee wallet trait methods to the real wallet implementation instead of panicking in swap-core tests.
• Update the Monero overflow assertion to match the current Amount::parse_monero error shape.

Reproduction

On current master, cargo test -p swap-core -p swap-feed -p swap-orchestrator reached swap-core and failed seven tests: the TxLock/funding tests hit the static-fee wallet stub, and parse_monero_overflows tried to downcast the current parser error into an old local OverflowError type.

Tests

• cargo fmt --all
• git diff --check
• cargo test -p swap-core --lib
• cargo test -p swap-core -p swap-feed -p swap-orchestrator
• cargo test -p bitcoin-wallet

I also attempted cargo test -p swap -p swap-asb; after initializing submodules and installing missing local cmake, it continued into the native Monero/Boost depends build and was stopped before Rust test execution because that path was no longer the shortest proof for this patch.

Bounty

Refs #724 / bounty: 0.25 XMR.

AI assistance

AI assistance was used for investigation and patch drafting; the changes above were inspected and verified with the listed local commands.