Fix auth integration improvements

[dorinengabdoh]

Description

This PR improves the test robustness and resource management for the authenticated electrum integration tests:

• Socket Cleanup: Adds explicit Shutdown::Both calls on the TCP streams when the proxy terminates to prevent thread and socket leaks.
• Graceful Docker Handling: Handles missing Docker gracefully in test_auth_with_keycloak so the test skips with an explanatory message instead of panicking on machines without Docker.
• Flakiness Protection: Increases wait_until_electrum_sees_block timeout from 6s to 15s to prevent flaky failures on slower CI runners.