fix: return query_timeout instead of closed on deadline

[Taure]

Summary

• When a pool deadline fires during query execution, the socket is closed and recv returns {error, closed}, which is indistinguishable from actual connection failures
• Now detects deadline-triggered disconnects by checking if the holder ETS table was deleted by the pool, and returns {error, query_timeout} instead
• Also handles {error, einval} which can occur in the same scenario depending on timing

Closes #96

Test plan

• New query_timeout test added to pgo_SUITE — verifies pg_sleep(5) with 500ms timeout returns {error, query_timeout}
• All 55 CT tests pass
• xref clean