EXP-1174: Expand connection resilience test coverage

[Steven Roomberg (sroomberg-ep)]

Summary

• correct the compatibility test proving BeanstalkConnectionError remains catchable as BeanstalkError
• verify ping() on a fresh client closes its probe without installing it as the active socket
• cover refused, timeout, socket, and DNS failures through ping() with host and port context
• verify all-down clients remain suppressed until positive backoff expires
• verify a flapping client is skipped before and retried at its backoff deadline

Tests

• 30 passed unit tests
• uv run --extra dev flake8 pystalk tests
• uv run python -m compileall -q pystalk tests

Ticket

EXP-1174