Add integration test harness for network-based client tests

[reez]

We need a small integration test harness before adding Electrum and Esplora flow
tests.

Scope

• add test/integration/ structure
• add shared helpers for setup and cleanup
• add env-based configuration for endpoints/credentials
• add skip logic when integration env vars are missing
• document how to run integration tests locally

Acceptance criteria

• test/integration/ exists with shared helper utilities.
• Integration tests can be gated by env vars.
• Missing env vars result in clean skips, not failures.
• Resource cleanup helpers exist for clients/wallets.
• A short run guide is added to README.md or test docs.
• Requirements for production-ready release #16, Add integration test coverage for Electrum sync/apply/persist flow #26, and Add integration test coverage for Esplora sync/apply/persist flow #27 link this issue.

Notes

• Keep this issue focused on harness only.
• Electrum and Esplora flow assertions stay in Add integration test coverage for Electrum sync/apply/persist flow #26 and Add integration test coverage for Esplora sync/apply/persist flow #27.