Make UNIX endpoint connection test deterministic

[samuel-williams-shopify]

The UNIX endpoint connection test used Thread#status as a readiness signal after making a separate bound? probe connection. This allowed the actual connection attempt to race with the accept loop and intermittently fail with ECONNREFUSED on macOS.

Test exactly one accepted connection instead, propagate exceptions from the server thread, and cover bound? independently. Endpoint#bind already starts listening, so the additional listen(1) call is unnecessary.

Verification:

• 100 consecutive focused runs with Ruby 4.0
• Full test suite: 101 tests, 186 assertions
• RuboCop on the changed test file